A Review of Train Protection Systems

Written by: Piers Connor, Felix Schmid and Charles Watson
and specially edited for Railway Technical Web Pages


Background - Automatic Train Protection Systems - Warning and Train Stop Systems in Use in the UK - Current UK Train Protection Systems - Warning and Train Stop Systems in Use in Continental Europe - Full Automatic Train Protection Systems in Use in Continental Europe - European Interoperable Railway Requirements - New Systems - Design, Delivery, Migration and Reliability - Future Strategy - A Business Case? - Bibliography

More articles on this site about signalling are listed in the Signalling Index


Railway signalling is the baseline safety system controlling the movements of trains. It is the safety critical part of the train control function of the railway. Once instructed by a signaller or an automatic system, it is responsible for setting up non-conflicting and safe routes for trains, for defining (safe) limits of movement and for transmitting instructions or commands to train drivers.

Traditionally, signalling systems in Britain and in many other countries have relied on the train drivers reacting to indications displayed to them by line-side semaphore or colour light signals and controlling the train’s speed in line with the instructions. During the 150 years of the use of railway signalling, drivers’ failures to respond to commands transmitted by signal aspects of any type can and have led to a number of accidents, some causing very large numbers of fatalities. In response to the continuing need to reduce risks created by train drivers failing to respond to signal instructions, various forms of driver warning devices and signal command enforcement systems have been developed. These have become known as Train Protection Systems. Those systems that continuously monitor actual train speed and enforce adherence to a commanded speed pattern are referred to as Automatic Train Protection (ATP) systems.

Types of Train Protection Systems

All types of train protection systems are based on the desire to reduce or eliminate the possibility of driver error resulting in a train movement related accident by failing to obey a visually displayed line-side or in-cab signal instruction. The development of train protection on main line railways began with the introduction of warning systems and subsequently progressed to enforcement of the instructions issued by these systems.

Originally, the warning systems alerted the driver that he or she was approaching an adverse or restrictive line-side signal aspect and required him or her to acknowledge the warning. Otherwise the systems would initiate a brake application after a short delay. Later developments by national railway administrations included various levels of speed limitation and enforcement. Also, some systems were expanded to cater for speed limits for permanent or temporary speed restrictions. Technologies adopted for such warning and train stop systems include combinations of permanent magnets and electromagnets, inductive polarity-changing responders, coded beacons and simply coded track circuits.

More recently, fully Automatic Train Protection (ATP) systems have been developed to enforce speed limits and movement authorities at the full range of restrictive signals, with and without line-side signals and including permanent and temporary line speed limits. Driving is still manual but speed limits are always enforced. Degraded modes though invariably include low speed driving on sight.

Two-Channel Safety Systems

Many older train protection systems are designed to rely on the statistical nature of driver and equipment failure. By designing the systems appropriately, it may safely be assumed that driver error and equipment failure will not occur simultaneously. A key characteristic of such systems is that the driver does not receive an indication of whether the train protection system is operating or not and is therefore encouraged to drive taking full responsibility for the movement of the train. The technical subsystem will only intervene if the driver attempts to pass a signal or to drive too fast. TPWS, train stop and Indusi are typical examples of this type of arrangement.

Automatic Train Protection Systems

Types of Balise

Passive Balise: Track based transponder that is ‘woken up’ by a low frequency signal and receives its energy from a passing train and then sends packets of information to the train.

Active Balise: Track based transponder that is powered from the signalling supply and that continuously sends packets of information to passing trains.

Passive and active balises can transmit either fixed or variable information or both. Many railways prefer to use balises powered-up by the passing trains. Simple location information is almost invariably transmitted by means of passive balises.

There are broadly two implementations of ATP systems – intermittent and continuous. Intermittent systems use electronic beacons (inductive or radio frequency) or short electrical loops positioned within the four-foot. These types of short-range devices are often referred to as "balises" (from the French word for ‘marker’). See Beacon Transmission for more information.  The continuous systems use a permanently active data transmission and monitoring system, either through electrical inductive coupling by means of track loops or coded track circuits or by means of radio transmission of limit of movement authorities.

Fully operational ATP systems were first introduced on metros in the late 1960s and are now common on such systems all over the world. Most metro applications use continuous systems in conjunction with automatic train operation. ATP was also introduced to the Japanese Shinkansen high speed route in 1964 and has since been introduced in various forms on a number of main line railways, often in conjunction with high speed train operation.


The basic defining principle of ATP is that train speed is monitored against the current permitted speed limit. The speed may be limited by line profile or signal indication, that is, the need to protect routes of other trains and track related constraints. If the allowable speed is exceeded, a brake application is invoked until the speed is brought within the required limit or the train is stopped.

Most ATP systems are based on conventional block signalling although these can be very short. Each block is described by a fixed dataset related to its location, length, gradient(s) and maximum (civil) speed limit(s). Each block will also have a variable data set derived from the signal aspects ahead and their effect on the resulting speed limit(s) for that block and the next block(s).


The speed limit on the approach to a restrictive signal forms a gradually reducing curve that follows the braking profile required to reach the target speed at the signal. If the signal shows a stop aspect, the target speed will be zero. The on-board monitoring equipment will continuously compare the train speed with the curve required to achieve the target speed and will initiate a warning – usually both audio and visual. If action is not taken, the system will cause a brake application.

In some implementations, a braking curve infringement calls for a full service brake initiation, in others for emergency brake. There are also differences in the brake release function. Some systems allow the driver to release the brake once the train speed has returned within the prescribed curve. In others, the brake command is irrevocable and the train must be brought to a stand before the driver can release the brake. There are also railway undertaking specific rules about the consequences when the ATP system has intervened.

Train Data

On the train, data comprising train weight, length, braking capability and maximum technically permitted speed are necessary to ensure compliance with speed limits set by the ATP system. Usually, the train consist data must be input by the driver before the trains starts its journey. Ways of validating this may be considered necessary.


In most cases, the performance of the equipment is monitored and recorded for further analysis in case of infringements or failures of the system. These systems are variously known as On Train Monitoring Systems (OTMS), On Train Data Recorders (OTDRs) or On Train Monitoring Recorders (OTMRs).

Drawbacks of Intermittent Systems

Continuous ATP systems allow constant data updates to be transmitted to trains so that the train driver can respond to changes in signal aspects as soon as they occur. Intermittent systems can only transmit changes in signal aspects when the train passes over a beacon or loop. This can restrict line capacity if a driver is unable to respond to a signal clearance, even though he or she can see the change of aspect, until the train’s on-board ATP computer has received a message from the balise located at the relevant signal.

In order to overcome this problem, infill loops or balises are provided at some signals (e.g., in 65% of the locations on the UK GWML system) to provide drivers with an update of a signal aspect and to allow brake release if a less restrictive aspect is shown.

Current UK Train Protection Systems

Automatic Warning System (AWS)

Following a Signal Passed at Danger (SPAD) accident in poor visibility at Harrow and Wealdstone in 1952 when 112 persons were killed, British Railways decided to deploy their Automatic Warning System (AWS) over the whole network to provide train drivers with an in-cab warning of the indication of the next signal. This was a non-contact version of a system originally used on the Great Western Railway. Following a long development and approval programme, widespread installation started in 1956. This system is still in use today.

AWS uses a pair of magnets, located in the four-foot about 200 yards (184m) in rear of the signal, to provide an indication of the status of the signal, while a receiver on the leading vehicle provides the input for the cab indications. A sound indication and a visual reminder system provide a warning for the driver that a stop or restricted speed signal is ahead of the train and a sound only indication is provided for a signal showing a green aspect. The driver must acknowledge the restrictive signal warning within 3s or the train brakes will apply automatically. AWS is provided on 98% of the UK main line railway network.

The use of AWS was extended to protect certain permanent speed restrictions after an accident at Morpeth in 1969, which was caused by a driver failing to reduce the train speed for a speed restriction. AWS was installed at speed restriction locations where the train approach speed is over 60 mph or the reduction in speed required is more than one third of the approach speed. This is mandated in a railway group standard.

After an accident at Nuneaton in 1975, the use of AWS was extended further to include temporary speed restrictions. It is now also used for emergency speed restrictions.

Although AWS has been partially effective in reducing train movement accidents, it has not eliminated SPAD or over-speed errors entirely. By design, it is only intended to provide an alert and a reminder of a restrictive signal aspect or speed restriction. As long as the alert is acknowledged, the driver may continue to drive the train at any speed. A number of accidents occurred where drivers forgot the restrictive aspect warning – despite the visual reminder in the shape of the so-called ‘sunflower’ device.

AWS may be considered to a limited extent as a fail-to-safety system since the main trigger element is a permanent magnet. Failure of the electro magnet results in a warning indication to the driver. However, removal of the permanent magnet from the track is not detected.

Driver’s Reminder Appliance (DRA)

The Driver’s Reminder Appliance (DRA) was introduced from 1998 to assist with the prevention of SPADs, particularly at station starting signals. It is not really a train protection device in the narrow sense of the terminology. The DRA device consists of a push button in the driver’s cab. The driver is required to operate this whenever the train is detained at a red signal. The button disables traction and prevents the driver from restarting the train until he has reset the button. The primary purpose is to prevent trains starting against a red signal when inadvertently given the "right away" by a conductor or station staff. The effectiveness of this system is a matter of debate because its operation may become ‘automated’ as part of the train starting sequence.

Train Protection and Warning System (TPWS)

To overcome the limitations of AWS, an enforcement system was developed for the British railway system, known as TPWS (Train Protection and Warning System). This is designed to enforce observance of restricted speed requirements and signal stops by imposing a full brake application when over-speed is detected or when a train is being driven past a stop signal. TPWS was first tried on a section of the Thameslink route in 1996 and was then installed across most of the UK network between March 2000 and December 2003.

For each signal or speed restriction equipped with TPWS, two pairs of electrical loops are placed between the rails. One pair is placed at the signal itself, the other pair some 200 to 450 metres on the approach side of the signal. Each pair consists of, firstly, an arming loop and secondly, a trigger loop. The loops are activated if the signal is showing a stop aspect.

The pair of approach loops, first met by the train 400 to 200 metres before the signal, are set between 4 and 36 metres apart. When the train passes over the arming loop, an on-board timer is switched on to detect the time elapsed while the train passes the distance between the arming loop and the trigger loop. This time period provides a speed test. If the test indicates that the train is travelling too fast, a full brake application will be initiated. In case the train passes the speed test successfully at the first pair of loops but then fails to stop at the signal, the second set of loops at the signal will cause a brake application. At the signal, both loops are adjacent so that, if a train passes over them, the elapsed time will be so short that the brake application is initiated at any but crawling speed.

TPWS does not provide the enforcement range of a full ATP system. If the train’s approach speed is too fast, TPWS will apply a full brake but the train may still overrun the signal if the speed is high enough. There is usually an "overlap" of 200 yards (183 metres) between the signal and any potential obstacle (train or points) in the block that it is protecting so there will be a much reduced risk of collision. With a possible total distance of 2000 feet (about 600 m) between the brake initiation and the block entrance, trains "hitting" the first loops at up to 120 km/h (75 mph) can be stopped within a distance sufficient to avoid a collision.

In an attempt to reduce the SPAD risks at certain high-speed locations, an add-on to TPWS, called TPWS+ is provided at certain signals where train speeds are above 100 mph. An additional loop pair is set about 770m in rear of the signal in order to provide the braking distance for a train "tripped" at 100 mph. Therefore, in these locations the signal approach has two speed traps. There are about 500 sites chosen on a risk assessment basis.

A further variation of TPWS, designed to be compliant with European requirements (q.v. below) and known as TPWS-E, was tried on a section of the GWML but it was not proceeded with further in order to allow rapid deployment of the already approved TPWS equipment.

TPWS is also provided at many (about 3000) Permanent Speed Restrictions (PSRs) to ensure that a train does not pass through the section of line at too high a speed. TPWS has also been provided at terminal platforms to ensure the train speed is reduced to 10 mph on the approach to the stops. This has had the effect of reducing capacity at some terminals because of the time taken for trains to clear the routes over the throat into the terminus. This type implementation may also encourage drivers to re-motor when travelling along a platform.

In spite of the limitations of TPWS, it is suggested in published data that 60% of potential accidents due to SPADs can be prevented by the installation of TPWS at critical locations. This has been achieved, it is said, at 10% of the installation costs of a full ATP system. However, this financial target was not achieved because of a decision to monitor the status of the TPWS beacons. With the lack of any in-built failure warning capability for TPWS, it was decided to link installations to the signal in rear, so that, in the event of a TPWS failure, the signal would display a red aspect. This addition to the original specification significantly increased costs.

TPWS does not replace the existing AWS system. AWS is retained, so that drivers still receive the warnings advising of adverse signal indications. The TPWS equipment was designed to interface with the existing on-board AWS equipment on trains or to replace it so that it could be fitted quickly.

Radio Electronic Token Block (RETB)

In rural areas of the UK, where long sections of single line require token block operation, a system for centralised control, using modern computer technology, was adopted. It is known as Radio Electronic Token Block (RETB).

Each train operating over the single line is equipped with a special speech and data radio transmitter/receiver with a unique identity. At the start of the single line, the driver stops and calls the control centre for authority to enter the section. If the line is clear, the signalman in the control centre transmits a coded "electronic token" data message which is received by the train and then shows the authority for that section on a cab display. The driver will then call for confirmation that he can enter the section. Once in the single line section, he will advise the control centre that he has cleared the loop track. A clearance marker board is specially provided to help him. When he has reached the end of the single line section, the driver calls the control centre again and offers to give up the token. After a "handshake" procedure by the control centre, he sends the token back by radio data transmission to release the section.

The signaller is provided with a computer system that allocates the coded tokens to each section and prevents more than one token being issued for an occupied section. It also receives the tokens sent back by each train as it reaches the end of the single line section.

At the exits of the single line sections, the points are permanently set in the direction of normal running and are "trailable" for trains entering the section, i.e. they allow a train to pass through at reduced speed using the wheel flanges to move the point (switch) blades aside reset to the normal position.

A "Distant Board" complete with AWS ramp, warns the driver that he must slow down for the movement over the points leaving the single line. The Points Indicator shows the position of the points. A "Stop Board" at the end of the passing loop warns the driver to stop and ask for permission to enter the next single line section. Stop board locations are provided with TPWS loops that are linked by radio to the signal controller. A "Loop Clear" board indicates to the driver when the rear of the train is clear of the points.

Induktives Sicherungssystem (Indusi, Tyne and Wear Metro, Germany and Austria)

Indusi is a German designed main line railway warning and supervision system used on the Tyne & Wear Metro. It is also standard in Germany and Austria. A track-mounted inductive transponder is used to transmit signal warning and speed limit codes to the train. The transponder is mounted on the sleeper ends just outside the four-foot, unlike most other systems where the transmitter is mounted between the rails.

The approach to a danger signal is protected by a transponder that indicates a maximum speed and causes emergency braking if a preset level is exceeded. On the main line version, an adverse distant signal indication must be acknowledged by the driver to prevent an emergency brake application.

The system is used in Germany for lines with a maximum speed up to 160 km/h and in Austria for line speeds up to 120 km/h. In the more recent electronic version, it includes speed supervision to a braking curve. It is not fully designed to vital standards.

Continuous Automatic Warning System (CAWS, Ireland)

Some sections of the main line routes in the Republic of Ireland and the whole of the line between Dublin and Cork are equipped with coded track circuits that provide in-cab signal indications. The system is known as the Continuous Automatic Warning System (CAWS).

The in-cab signal indications repeat line-side indications and are accompanied by an alarm buzzer when there is a change to a more restrictive aspect. The driver is required to acknowledge the alarm within 8s to prevent an irrevocable automatic emergency brake application. After the operation of the emergency brake, there is a two-minute delay before the system can be reset and the train is allowed to proceed.

The system is not vital in that the driver can acknowledge a restrictive signal warning and can then allow the train to proceed without reducing speed.

Train Stops (Trip-Cocks, London Underground)

LUL uses mechanical train stops combined with fixed blocks and individually calculated signal overlaps to provide train protection on most of its lines. The system prevents collisions by providing an individually calculated full speed braking distance beyond every stop signal so that a train "tripped" by the train stop will come to a stand without infringing a restricted block. Trains are restricted to 10 mph for 3 minutes after being tripped to enforce driving on sight at caution speed. This is known as SCAT (Speed Control After Tripping).

Degraded Operation

None of the systems mentioned is used for continuous speed supervision and all of them can be isolated in the cab and the train can be driven at normal speeds regardless of signal aspects. Most of the systems require a positive action to issue a warning or restrictive data. However, TASS displays some of the behaviour of a true ATP system in that it can detect missing balises.

As mentioned before, in the case of TPWS, the transmitters at a location are linked to the signal in rear so that this signal will show a red aspect in the event of TPWS failure at the next signal. This is because passing trains cannot detect failures of the track-mounted equipment.

Whilst the described systems above all provide some protection against collisions and over-speed derailments, none provide the full and vital protection that is available from modern Automatic Train Protection systems.

Automatic Train Operation with Train Stops (Glasgow Subway)

The Glasgow Subway has fixed blocks divided by stations. Each block stretches from station starting signal to station starting signal. There are no intermediate signals except at the depot connection. A recently replaced ATO system uses Siemens equipment with track-mounted beacons and on-broad processors that control the train driving and braking functions. Each station is provided with 2 approach beacons for the ATO profile requirements and a third start beacon in the platform that is linked to the starting signal and provides authority to proceed to the next station. The system software is designed to SIL Level 2.

Train protection is provided by contactless train stops provided at each signal. The equipment was supplied by SAGEM. The track mounted device consists of a permanent magnet supplemented by additional induction coils to indicated a proceed signal. Absence of the induction signal will trigger a train brake command.

If a train fails to respond to the ATO commands (or a train in manual mode passes a signal at danger) the on-board receiver will trigger an irrevocable emergency brake. The driver is required to operate a reset button to restart the train and speed is limited to 25 km/h until another start beacon is passed in the "clear" position.

Automatic Train Control (ATO and ATP, London Underground)

On the Victoria and Central Lines, full Automatic Train Protection is provided by two different Automatic Train Control systems that also include Automatic Train Operation. ATP failure enforces manual driving with speed of movement reduced to 10 mph. The system on the Victoria line was introduced in 1967 and has been partially upgraded twice.

Both systems use continuous data transmission through coded track circuits but they are each unique to the line upon which they operate. The Victoria Line system uses four codes while the newer Central Line system uses 13 codes.

BR-ATP (Two Versions)

British Rail installed two Automatic Train Protection systems with full speed supervision for trial purposes in the early 1990s, one on the Great Western Main Line (by ACEC Belgium – now Alstom) and one on Chiltern Railways (Selcab by Alcatel) between Marylebone and Aynho Junction. Both are intermittent systems with infill loops, added to allow early release of the braking demand and its supervision when signal aspects change.

The information transmitted to the train consists of signal aspect, routing, applicable speed restrictions, the distance to the next signal and gradients. Drivers are shown the permitted train speeds by LEDs displayed around the circumference of the cab speedometer. Green LEDs show the target speed while yellow LEDs show the permitted release speed. Information on the number of signals to the next red and speed restrictions is also displayed.

The drivers set up the systems using train data input unit in the cabs that interface with the vehicle computers. The systems generate three speed curves, one for movement authority, a warning curve and an intervention curve. Each is separated by 3 mph. If the train exceeds the warning curve speed, the driver gets an audio/visual warning. If the speed reaches the intervention curves, the brakes are applied.

Although the systems were introduced as a trial they are still operational and, since the Southall accident, it has been the policy for both train-operating companies that a train will not be allowed to enter service unless the ATP system is operational.

Tilt Authorisation and Speed Supervision (TASS)

TASS (Tilt Authorisation and Speed Supervision) has been introduced on the West Coast Main Line (WCML) in order to allow tilting train to operate safely within the somewhat restricted UK railway infrastructure gauge. The primary purpose of TASS is to ensure that a train is prevented from tilting where clearances between adjacent trains or between trains and infrastructure are restricted. TASS also imposes line speed limits for equipped trains depending on whether or not the tilting system is operational.

The TASS system is installed on the Virgin Pendolino Class 390and Super Voyager Class 221 fleets and is designed to European Rail Traffic Management System (ERTMS) standards. Data is transmitted to the train by track mounted "Eurobalises" and collected by an antenna mounted under the leading vehicle. Speed limits are different for the two classes of tilting trains.

The speed limits for tilting trains are displayed on line-side signs alongside the signage for non-tilting trains. As the train passes over the first TASS balise, the driver is shown an indication light to verify the operating status of the system. Each TASS balise transmits to the train-borne equipment the position of the next balise, thus ensuring a continuous ‘daisy-chain’ of supervision. A further indication shows when tilt is enabled. The driver is responsible for driving within the correct speed throughout the trip. AWS and TPWS are provided since TASS does not sense or transmit signal aspects.

Over-speed is indicated by an audio-visual alarm and, if speed is not reduced, the train brakes are applied automatically. The brakes are released and the alarm can be reset when the train speed is reduced to the correct level. Where clearances are restricted, the train automatically stops tilting but the speed is still monitored by TASS. The train speed must be reduced by 25 mph, that is, conventional operation, if tilt fails or a balise is not detected.

Docklands Light Railway

The Docklands Light Railway (DLR) uses an ATP system with full continuous speed supervision known as Seltrac, supplied by Alcatel of Canada. Seltrac is a transmission-based, automatic train control (ATC) system, combining both automatic train protection (ATP) and automatic train operation (ATO). The duplex transmission system is via a continuous track mounted cable, with loops crossovers at 25m intervals forming train position validators. Effectively, the system provides pseudo moving block capability thanks to very short ‘virtual’ blocks. Train detection and inductive data transmission between track and train are effected by means of the cable. A fixed block back-up system uses axle counters for train detection.

Trains are operated fully automatically without driver intervention but a train captain is provided on every train and can operate the train at reduced speed in an emergency. A Vehicle On Board Computer (VOBC), linked to the transmission system, controls the on-board vital and driving functions. All trains are controlled by Vehicle Control Computers (VCCs) from a central control facility.

The system has a good safety record but it requires continuous track cables and uses a back-up axle counter train detection system. It is therefore relatively expensive to install and maintain. As with any system requiring extensive track-mounted cabling, there is the risk of damage to the cables during track maintenance activities. This type of system can only be considered for a metro-type operation with a high service frequency.

Transmission Voie-Machine 430 (TVM 430)

Phase 1 of the Channel Tunnel Rail Link (CTRL) is equipped with the French continuous transmission ATP system known as TVM 430. This is also the system used in the Channel Tunnel and the system that will be used on Phase 2 (except for the station area at St. Pancras International. TVM 430 ("track to train transmission") is a cab signalling system used on the more recent TGV lines, developed from the earlier TVM 300 system by the French company CSEE.

With TVM 430, the line is divided into fixed blocks from 500 to 1500m long. The length of the block depends on the location, civil track speed limit and the train capacity required in the area. Line-side marker boards indicate block boundaries visually. Each block carries a speed code that is injected into the rails as part of the track circuit current and transmitted to the train as it progresses through the block. There are five standard codes representing speed limits between 0 km/h and 300 km/h. The codes are generated according to the condition of the route ahead, that is, the distance to the next ‘obstacle’. In the case of a failure, the driver can "drive on sight" up to a maximum speed of 35 km/h.

The driver is presented with the codes in the cab display with the target speed at the end of the current block and the target speed at the end of the following block. The target speed is the speed at which the train should exit the current block and enter the next.

In older versions of TVM, the target speed indication for the driver was updated only at every block boundary, resulting in a stepped speed profile. With TVM 430 the train has a continuously varying target speed through calculations by the on-board computer, giving a much more realistic speed profile for the driver to follow.

Eurostar trains are provided with a system of network codes in order to allow the train to comply with varying line speed limits over different routes. On lines where the maximum speed limit is 300 km/h (186 mph), a different network code is used from that used on the section through the Channel Tunnel, where the speed limit is 160 km/h (100 mph).

TVM is a safe, reliable and well-proven system but it relies on track circuit based continuous transmission technology and is therefore expensive to install and maintain.

Warning and Train Stop Systems in Use in Continental Europe

Crocodile (France)

This is a French designed AWS system very similar in concept to the UK AWS. The name derives from the corrugated appearance of the track-mounted equipment. It is officially described as Brosse Repetition Signal (BRS). BRS is installed on all main lines of SNCF, SNCB and CFL. A brush on the train contacts the track-mounted device as it passes. +/- 20V from a battery is supplied to the Crocodile, depending on the signal aspect. There is a bell and flashing light indication to the driver and, if not acknowledged, an automatic brake application occurs.

Crocodile only acts as a vigilance system. Crocodile is less safe than AWS since absence of voltage cannot be detected. It does not provide any warning to the driver if it becomes defective. It may now be considered as outdated.

ASFA (Spain)

ASFA is a cab-signalling and train protection system widely used in Spain. Intermittent track-to-train communication is based on magnetically coupled resonant circuits in such a way that nine different sets of data can be transmitted. A resonant circuit trackside is tuned to a frequency representing the signal aspect. The system is not fail safe, but reminds the driver of the signalling conditions and requires him to acknowledge restrictive aspects within 3s. Lamp and bell warnings are provided for the driver.

Three different train types can be selected on-board to give continuous speed supervision of line speed and after passing a restrictive signal (160 km/h or 180 km/h). A speed check can be carried out (60 km/h, 50 km/h or 35 km/h, depending on train type) after passing a transponder 300m before reaching a stop signal and a train trip is provided at signals at danger. There is an irrevocable emergency brake upon violation.

Automatische Trein Beïnvloeding (ATB EG, Netherlands)

The Dutch railways ATB system appears in two basic versions on Dutch railway lines - ATB EG and ATB NG. ATB-EG ("ATB Eerste Generatie": ATB first generation) is the original continuous system and ATB-NG ("ATB Nieuwe Generatie": ATB new generation) is a new intermittent system designed for speeds up to 360 km/h – see next section.

ATB EG is installed on the vast majority of lines of ProRail (the new Dutch infrastructure authority) and is a fail-safe system using coded track circuits of conventional design and two versions of on-board equipment, ACEC (computerised) or GRS (electronic). The transmission between coded track circuits and on-board equipment is via vehicle-mounted induction pickup coils suspended above the rails.

There are six speed codes (40, 60, 80, 130 or 140 km/h) that are picked up by the train and displayed to the driver. There is a gong warning at a code change and a bell if the system requests a brake application. The emergency brake is invoked on over-speed or if the driver does not react to an acoustic warning.

The system does not distinguish between the 40km/h limit and stop. This allows driving on sight in the event of an equipment failure.

Transmission Balise Locomotive – (TBL, Belgium)

TBL is available in two versions, TBL1 and TBL2. TBL1 simply provides an advance indication of the signal aspect followed by an emergency brake application plus a train trip function for signals passed at danger. Data is supplied through track-mounted loops. The TBL loops require an external power supply, unlike most other balise systems.

BACC (Italy)

BACC is installed on most of the infrastructure of RFI (Rete Ferroviaria Italiana) in two versions, both of which operate in a similar way. Conventional coded track circuits operate at one of two carrier frequencies to deal with two train classes – those operating either above or below 180 km/h. The transmission between coded track circuits and on-board equipment is via induction pickup coils suspended above the rails.

For the <180 km/h trains a 50Hz carrier frequency is used for 5 amplitude modulated (AM) speed codes. A 178 Hz carrier frequency with 10 AM speed codes is used for higher speed trains. In the cab the driver sees the speed corresponding to the code and the signal aspect. The emergency brake is applied upon over-speed but can be released when the speed is reduced to the approved level. The version of BACC used on the Rome to Florence high-speed line is effectively an automatic train protection system with full speed supervision.

Full Automatic Train Protection Systems in Use in Continental Europe

Automatische Trein Beïnvloeding (ATB NG, Netherlands)

The ATB NG system was introduced to the NS in the mid-1990s to implement full ATP and to eliminate the expensive and maintenance intensive coded track circuits. It consists of track mounted balises and on-board computing equipment. An infill function based on a cable loop is also available. The data transmission is between the active balise and an antenna on-board. The system is direction sensitive, so the balises are mounted between the rails with a small offset from the centre line. ATB NG on-board equipment is fully interoperable with the original ATB EG trackside equipment.

To initialise the system, the driver must input train length, maximum speed and brake characteristics. The driver has a display for maximum line speed, the target speed, the target distance and the braking curve. The line speed, speed restrictions, the target stopping point and the brake performance are monitored. Advance warning audio and visual alarms of over-speeds are provided. The emergency brake is invoked if movement supervision is violated or the driver does not react to a warning.

Ebicab (Sweden, Norway, etc.)

Ebicab is the standard ATP system in Sweden, Norway, Portugal and Bulgaria. Identical software in Sweden and Norway enables cross-border train operations without changing drivers or locomotives, despite the differences in signalling systems and rules. Software variations are used for the systems in Portugal and Bulgaria.

There are two versions of the system, Ebicab 700 and Ebicab 900, both providing similar safety functions. The system consists of signal encoders sending data to balises mounted in the middle of the track that are in communication with on-board computerised equipment. The data transmission takes place between passive balises (two to five per signal) and an antenna under the vehicle that also powers up the balise as it passes above it.

Inputs by the driver include the usual train parameters plus a facility to advise the train-borne computer of adverse railhead conditions. Displays show the driver the maximum line speed, target speed and advanced information for up to five blocks’ worth of speed restrictions beyond the first signal. The time to a service brake intervention is also shown if an over-speed is predicted, together with three warnings.

Service braking is initiated when the train speed exceeds the permitted speed by 10 km/h (5 km/h for Ebicab 900) but this is released when the speed has returned to the permitted level. If an emergency brake is initiated by failure to maintain the required deceleration through the service brake, the train must come to a stand before it can be released. A default speed of 40 km/h is allowed for trains passing a signal under authorisation or to cope with failure modes.

KVB (France)

This is the standard ATP system used in France and is technically similar to Ebicab, although the information provided on the driver’s display is more limited. The system is installed on the conventional railway network, in particular on the routes where high speed trains (trains à grande vitesse or TGV) approach major termini. Some sections of the high-speed lines (ligne à grande vitesse or LGV) use the system in place of or together with TVM for certain spot transmissions and for the supervision of temporary speed restrictions where appropriate speed levels are not available from TVM codes. Spot transmissions include such items as door release authority, overhead line section switches and radio channel changes.

The system is overlaid on the conventional signalling system. The data is transmitted inductively between passive balises (between two and nine may be required per signal) and the on-board antenna which activates (powers up) the balise as it passes. The data transmission capacity is limited, hence the need for many additional balises.

The driver must input train data unless the train is a modern, fixed formation unit, where data is automatically programmed into the on-board supervision computer. In the event of any over-speed, the driver will receive a warning and then an irrevocable emergency brake will apply. The brake cannot be released until the train is at a stand.

This system is provided on Eurostar trains and it is proposed to use it on certain limited sections of the CTRL, most notably on the approach to St. Pancras station.

TBL 2 (Belgium)

TBL 2 is used on all lines in Belgium where the permitted line speed is greater than 160 km/h. TBL 2 is a cab signal system and is similar to the UK GWML ATP system (see under Britain) which uses a powered balise in the form of a steel loop with additional, long infill cable loops provided to give early warning of signal indication changes. TBL 2 is direction sensitive. This capability is provided by having the balises mounted between the rails with a small offset from the centre.

The driver is shown the maximum allowed speed (from a braking curve), the target speed, the target distance and the train speed. Audio-visual warnings of violations are provided with an emergency brake in the event of no acknowledgement.

Linienförmige Zugbeeinflussung (LZB)

This is a continuous ATP system installed on all lines in Germany where speeds exceed 160 km/h. It is also installed on some lines in Austria and on the Spanish high-speed line (alta velocidád or AVE) between Madrid and Sevilla. The on-board equipment is normally integrated with the German Indusi system and incident recorders. Data transmission is via inductive cable loops and on-board ferrite antennae. One conductor of the loop is clipped to the foot of one of the running rails while the other is fixed in the middle of sleepers. The two conductors are crossed over at regular intervals to allow the trains to obtain an accurate reading of position and speed thanks to the 180° phase shift.

The driver inputs train length, maximum train speed and train braking characteristics. A two-needle speedometer displays the maximum permitted speed and actual speed. A separate linear display provides the operating mode, the status of data transmission as well as the target speed and the distance to the target (limit of movement authority).

The on-board system monitors line speed including temporary and permanent speed restrictions, maximum train speed, the target stopping point, the train’s direction and the dynamic speed profile. It is also capable of including auxiliary functions, such as circuit breaker and pantograph operation. The emergency brake is called if the speed supervision is violated. The brake can be released when the train speed is restored within limits.

The system can be used in vital automatic train control connected with automatic motor and brake control. Line-side signals are not required for LZB equipped trains. Drivers of such trains are instructed to ignore wayside signals that are provided for non-equipped trains. LZB is the earliest continuous automatic train protection system that is not track circuit based, to the best knowledge of the authors of this report.

European Interoperable Railways

European Interoperability Directives

The question of the application of full ATP on UK railways cannot be separated from the requirements for European interoperability. It would not be legal or cost effective to pursue a policy of ATP installation on the UK network without incorporating the technical requirements for interoperability. Rolling stock and signalling suppliers in the UK are already installing equipment that is compliant with or capable of upgrading to the standards required by European interoperability directives and the Technical Standards for Interoperability (TSIs).

The European interoperability requirements are enshrined in the European Rail Traffic Management System (ERTMS). ERTMS has been developed to support the implementation of two European 'interoperability' directives: 96/48/EC for high-speed lines and 2001/16/EC for conventional services. These offer guidelines for the adoption of technical standards across the EU for rolling stock, traction current and signalling and control systems. The technical requirements and UK national guidelines for high-speed lines have already been issued and those for conventional railways are expected shortly.

The routes requiring interoperable systems for high-speed lines are defined in the Trans European Network (TEN). Four UK high-speed main lines are included in the TEN:

The West Coast Main Line

The East Coast Main Line

The Great Western Main Line and

The Channel Tunnel Rail Link

The European Rail Traffic Management System (ERTMS) is a fundamental building block for the implementation of interoperability on the TEN. The European Train Control System (ETCS) covers the physical signalling and train control part of ERTMS.


The ETCS specification has three substantially different ATP operating levels allowing a stepped transition from conventional line-side signalling to a full moving block concept, with some incremental additions. The levels provide full speed supervision and varying amounts of in-cab information, throughout a train’s journey, and may be summarised as follows:

Level 1 – No Infill (System A)

The Level 1 system is overlaid on existing visual signalling systems. It is based on Eurobalises providing ATP supervision and protection. It can have an adverse effect on route capacity, in a range of 5-15%, generally, because of trains having to reduce the speed as demanded by the supervision curve before receiving new information at the next signal position.

Level 1 – With Infill (System B)

This level uses infill signal indication updates between the main balises, either by means of additional balises, using so-called EuroLoop or by radio. The driver thus receives updated information on the status of the next signal(s) before reaching the main balises. This option reduces the adverse effect on route capacity, but is more expensive to install and operate, as additional line-side equipment is required.

Level 2

Level 2 uses conventional, fixed block train detection (with track circuits or axle counters) but the driver receives all train movement authority information via bi-directional GSM-R radio. It has full continuous ATP supervision and protection with in-cab signalling displays. It can improve route capacity and line-side signals can be removed. With signals retained it is referred to as System C and with signals removed as System D.

Level 3

Fixed blocks are not required for Level 3. This level uses balises and tachometry to detect train position. All conventional signalling is removed and all data communication is via GSM-R radio. It can improve route capacity but, although technically feasible, it has not been proven on an existing main line railway and, in our view, it currently represents a significant risk in terms of operability and reliability. These issues have yet to be resolved.

Current ETCS Development Status

A number of ETCS installations have been completed or are still being introduced on sites across Europe. In the UK a trial installation has started operations on the Cambrian Line in Wales. The project is two years late and there have been a large number of teething problems. The signs of trouble had already been seen in other installations.

In Switzerland, the Zofingen – Sursee route was re-signalled with conventional colour light signals on completion of the trials there because a number of issues arose which provided pointers for the future development of ETCS. These can be summarised as follows:

  • The evolution of standards during the design and installation stage led to a potential lack of interoperability with later installations;
  • Additional time was required for the development, testing and commissioning, partly because the complexity of converting existing trains had been underestimated;
  • The complexity and range of training for drivers, technicians, signallers, controllers and managers was substantial;
  • An assistance hotline for drivers with operating or technical problems en route had to be provided;
  • Personnel for 24 hour system monitoring had to be provided;
  • Trains had to be double manned during a burn-in period;
  • Failure rates during early operations were high, largely due to insufficient processing speed and poorly designed databases;
  • The lack of hot-stand-by equipment caused some unnecessary breakdowns.

The new Swiss high-speed line between Mattstetten and Rothrist came into operation in December 2004 but, at first, it had to be operated at 160 km/h rather than the planned 200 km/h because the new ETCS Level 2 system was late being brought into service. The railway authorities had thoughtfully predicted this and provided line-side signalling as a fallback.

This last point may be crucial in any cost/benefit analysis where operators insist on the provision of a back-up system in case the ETCS fails. Experience with radio-based or electronic ATP installations applied to existing railways suggests that this is likely to be a major issue.

United Kingdom Development

The Cambrian Line project involves the equipment of 14 trains and 220 route-km of railway with full ETCS Level 2 equipment, RBCs and GSM-R communications. The existing Radio Electronic Token Block (RETB) and line-side signals will be removed once the ETCS system is fully working. Cab signalling has been provided on the trains (Class 158 DMUs and Class 37 Locomotives). The system became operational in 2010.

Future ETCS Development

The programme for the development of the Cambrian Line test installation of ETCS Level 2 originally identified the timetable change 2008 as the start of full operation but the programme is two years late.  Assuming that the system beds down in time, it is expected that ERTMS will be specified for all new resignalling on main lines. In the intervening period, project preparation, surveys, scheme plans and so on could be formulated for the first major route to be equipped.

The objective is to fit the UK network with ETCS Level 2 or 3 wherever a business case can be made. In a number of areas this could involve conventional lines, community railways, including those requiring operating subsidy.

It should be noted that there is scope within the regulations for derogations to the requirement for fitting of ETCS compliant ATP, particularly where it would not be economically viable. In view of the fact that Britain has TPWS, making an economic case for ETCS is going to be a struggle.

Global System for Mobile communications (GSM-R)

The introduction of ETCS Levels 2/3 is reliant on the installation of GSM-R as the data and speech carrier. The testing and installation of the system for UK ETCS is running in parallel with the Cambrian Line trials. The UK train radio system and fixed telephone network are also being upgraded. Any failure of the GSM-R project to maintain parallel implementation with ETCS will delay the implementation of train control interoperability in the UK. There is some suggstion that bandwidth is an issue already for GSM-R and that there's no capacity in the existing equipment for anything more than normal traffic. Emergency traffic might overload the system.

GSM-R or satellite based train control systems will still require some ground based validation (passive Eurobalises) and train detection through track circuits will very likely be required for turnout locking and in complex junction areas.

Conventional (Community) Railways

The ETCS technical requirements for conventional railways are not yet published but it is very likely that the equipment will be similar to and compatible with that required for high-speed lines. This will allow trains to move freely between ETCS fitted high-speed lines and community railways without the need for dual fitting of different systems.

It is inevitable that existing conventional railways will continue to use the systems that are currently standard in the country of origin and that the progress towards commonality will be slow. Both the high-speed and conventional interoperability regulations drawn up by the British authorities require that new rolling stock and signalling equipment be designed and built to accommodate ETCS equipment in addition to any "legacy" equipment needed to operate on the existing railway network. This has already been achieved on the Eurostar fleets and, in principle, therefore could be applied to existing UK fleets.

Conventional lines should be converted to ETCS as systems are due for renewal or where the use of communications based train control can be shown to reduce substantially the operating and maintenance costs.

Metros and Light Railways

Although most metro systems worldwide are already fitted with more or less advanced train protection system and risks are generally low, there are European Union moves to standardise a unified European Urban ATP system. Implementation of this is likely to be a long-term objective since most operators have their own standards. The benefits of the adoption of a unified metro train protection might seem to be small but could lead to substantial reductions in replacement costs.

Questions of interfaces with main line railways must be addressed. In the UK this relates particularly to London Underground and the Newcastle Metro. Both systems share lines with Network Rail and must be considered in the context of interoperability for community or conventional (non-high speed) railways. There will also be the question of electro-magnetic interference on shared and adjacent routes, which will require a separate study for each route. This will also apply to light rail routes adjacent to main line systems like those in Wimbledon (Croydon) and Nottingham.

New Systems

Communications Based Train Control

Much of the development effort in the area of new signalling systems is concentrated on removing reliance on track-based equipment. Generically, this type of technology is referred to as Communications Based Train Control (CBTC) and is broadly similar to ETCS Levels 2 or even 3 in as much as conventional track circuits are not necessarily required for train detection. Where trains do not report their own location using tachometers, Doppler radar and balises, detection can be provided additionally by combinations of track circuits, or by means of Global Positioning by Satellite (GPS) or Global Navigation Satellite Systems (GNSS) as they are now called. Some systems reduce or eliminate the need for fixed track based systems like track circuits and axle counters and rely instead on a combination of train borne speed and location data together with balises and GNSS.

Some CBTC systems use radio-based data bearers to transmit train locations to the central control computers. These are now being used on some metros and people-mover systems like the Las Vegas Monorail. However, some track-based data is still usually considered essential in locations where track separation for parallel tracks and turnout (point) locking is required.

In the US, a system using GNSS is operating over part of the route between Chicago and Detroit but it retains the existing line-side signalling for use by non-equipped trains. Another GNSS-PTC (Positive Train Control) system is due to enter service this year between Mazonia and Springfield, Illinois. A new feature is that it uses on-board sensors to detect a train’s route through junctions (point-work).

The Alaska Railroad has placed an order with Union Switch & Signal for a complete PTC system using vital GNSS equipment without balises. Similar systems are being developed for use in Australia and Brazil. In Europe, a number of trials are taking place to test the use of GNSS.

It should be noted that some of the systems using satellite links for train separation were designed in the first instance for areas of "dark territory" where line-side signals are not provided and trains are controlled by train orders and track warrants. The CBTC systems are used to increase throughput and crossing point efficiency whilst not reducing safety.

There is value in considering such systems for use in the UK, particularly where remote routes with low train frequency might have to be closed without a reduction in infrastructure replacement and maintenance costs. GNSS based systems can also be used for passenger information both on trains and at stations.

A further possibility for the use of GNSS is to trigger the warning on the approach to a level crossing with a constant-time lapse regardless of the speed of the train. In a similar way, track workers could be alerted to the approach of a train within a known fixed time.

Distance To Go – Radio (DTG-R)

The Westinghouse Rail Systems DTG-R (Distance To Go- Radio) system about to be installed on the London Underground’s Victoria Line is likely to become the first instance of a full ATP application using radio-based data bearers in the UK. The system is based on fixed block train detection with vital radio data transmission between each interlocking area and the train. It will be used in conjunction with ATO to replace the existing induction based transmission system used on the line.

The line is divided into what Westinghouse engineers refer to as "interlockings" even where only plain line is provided. An interlocking usually covers a station and the tracks halfway to the next station. Block occupancy data is collected by the interlocking and passed to a Fixed Block Processor, which converts the data into a radio message. The radio message transmits the block occupancy and proceed status to the train. Geographical data on the line profile and PSRs is carried on the train. Train positioning is updated by track-mounted balises.

Trains will operate under ATO but will be capable of operating under full ATP protection with manual driving. The ATO system will be similar to the existing Central Line installation using loops known as Platform ATO Communication (PAC) loops. The failure mode will restrict manual driving on sight to 10 mile/h as on the existing system.

Design, Delivery, Migration and Reliability

Key Issues

There are a considerable number of technical and operational problems to be overcome before installation of ETCS compatible ATP and migration towards full operation of ATP on the running railway can be achieved. Promoters of any future legislation must be aware of these and of what can be done realistically before any new regulations are introduced. The issues listed below will affect the credibility of those affected as well as the regulators themselves.

  • System and component approvals;
  • Line capacity;
  • The wide variety of existing rolling stock to be fitted with new systems and the effort involved in the physical installation;
  • The interfaces between on-board systems and new ATP equipment;
  • The interfaces between new ATP equipment and existing signalling and control equipment;
  • Mixed system operation;
  • New operating rules;
  • Conflicts between manufacturers over intellectual property rights;
  • Reliability and maintainability;
  • Failure modes and degraded operations;
  • Interfaces between railway systems (e.g., high-speed and community lines, community lines and metros); Phasing of migration;
  • Signal engineering skills shortages.


Using lessons learned from previous introductions of new train protection systems, both in the UK and continental Europe, there is an opportunity for the approvals process required for new ATP systems to be implemented smoothly and such systems to be deployed rapidly. Operators, owners, suppliers and installers should be encouraged to interface with the regulatory authorities as early as possible.

In looking towards future developments, there is a risk that accepted standards in place today will reduce the drive to improve and develop new systems because of changes required to existing standards. The process of changing and getting approval for compliance with new standards could act as a deterrent to improvements unless standards are carefully written to allow improvements.

Line Capacity

It is a widely held view in the industry that the introduction of ATP will reduce line capacity. This is not necessarily the case in all areas and there are positive benefits from the use of ETCS Level 2/3 in terms of train throughput.

In simple terms, a train’s speed on the approach to the block will have to conform to the safe braking distance profile. However, there is no reason why ECTS Level 2 should not be capable of supporting the existing capacity and, indeed, improving on it using shorter fixed blocks. As it does not require fixed blocks, Level 3 will improve capacity in most areas.

Retrofitting ATP to Rolling Stock

In respect of retrofitting ATP equipment to existing rolling stock, the lessons of the fitting of BR-ATP to the HST Class 43 First Great Western fleet and the Chiltern Railways Class 165/168 DMU fleet should be clearly understood and widely disseminated to operators, suppliers and regulators. These lessons resulted in the need for some time-consuming and expensive measures:

  • Overcome underestimated fitting requirements;
  • Rewire parts of rolling stock;
  • Overcome drivers’ resistance;
  • Increase reliability;
  • Rectify intermittent failures in an intermittent transmission system;
  • Reduce equipment self-test and boot-up time;
  • Increase maintenance requirements;
  • Increase and repeat training;
  • Manage TSR and ESR implementation;
  • Manage adhesion issues.

It is recommended that a group with a mandate to address lessons learned by both UK and overseas organisations concerned with the design, manufacturing, testing, commissioning, approval and operation of ETCS should be set up and maintained to act as a reservoir of knowledge and experience with the objective of reducing the problems associated with the implementation of new and enhanced ATP systems.

There will doubtless be some rolling stock whose age precludes a cost-effective case for fitting new ETCS equipment and which will continue to operate with legacy equipment.

Skills Shortages

Any railway signalling developments proposed for the next 10 years will depend on the availability of suitable engineering and installation skills. A large proportion of the relevant UK skills base will be absorbed by the London Underground PPP contracts that cover the re-signalling of over 300 route-km of London Underground lines in the period 2006 to 2017. The equipment to be supplied is novel, ATP based throughout, with complex performance requirements and is still in development. Existing skills shortages in the UK are forcing the contracting companies to source part of the project staff from overseas. Modernisation of the UK main line signalling infrastructure with ETCS based equipment will require similar skills. Shortages, if not addressed immediately, are likely to slow down all replacement programmes.

Operating Rules

Adapted and new operating rules will need to be written for the new train protection systems introduced to the UK network. Amongst other issues, these rules will have to cover:

  • Normal operations, e.g., driving with or without line-side signals under ETCS Levels 2 and 3;
  • Failure modes and required actions, e.g., loss of radio contact, failure of on-board equipment;
  • Protection of disabled trains;
  • Possessions of the track by the maintainer;
  • Starting and stabling of trains;
  • Movement of trains within station limits;
  • Shunting.

Many of these processes will have to change significantly compared with existing practices and will, during the migration periods, have to be run in parallel with them. These circumstances could cause serious delays to train operations or present significant safety risks and should be included in any assessments carried out to determine the methods for introducing ATP.

In particular, the man-machine interface must not be forgotten. The success and acceptance of ATP systems will depend to a substantial extent on drivers’ ability to understand and act upon the normal operational modes and failure modes possible with complex ETCS and similar ATP systems. Areas for further exploration concern the man-machine interface, the requirements for revoking authority (replacing signals to danger) and driver supervised movements.

Roll Back Detection. 

To provide complete protection, any ATP system adopted will have to incorporate some form of roll-away detection, including roll-back. If this is to be fitted to existing rolling stock, further interface issues will have to be resolved.

TSRs and ESRs

In any ATP system, the requirement to impose temporary or emergency speed restrictions (TSRs and ESRs) must be considered. In theory, this would seem simply a matter of placing a passive balise on the approach to the speed restriction with its required speed suitably pre-programmed. However, the issue of gradient effects and how this would be calculated and programmed into the balise must be considered. The quality control of such programming is vital and suitable methodologies must be agreed and monitored. In any case, there is a need to stipulate a standard default mode for temporary balises.


A range of complex technical and operational issues will need to be considered as part of the development of migration plans for each area to be upgraded to ATP. These will include:

  • Technical interfaces;
  • Operational changes (see rules above);
  • Electro-magnetic compatibility;
  • Integration with existing systems;
  • Retrofitting to existing rolling stock.

Looking at the existing installations of signalling equipment, both at the line side and on rolling stock, it is likely that new ETCS equipment will have to operate alongside existing systems for at least 30 years.

Degraded Modes – Effects of Failures on Safety and Operations

There are a considerable number of issues related to the effects of failures and the resulting degraded modes of operation. A careful balance must be found between ensuring train movements can continue whilst risk is minimised when operating trains under degraded modes of ECTS and other advanced forms of train protection.

Degraded modes will include the initiation of a brake application by the equipment because of driver error as well as equipment malfunction. A clear distinction should be made between the two but it may be necessary to impose operating rules that assume driver error even in the event of equipment failure. Detailed studies and consultation with regulators and operators are required to ensure the correct rules are understood and agreed prior to equipment installation.

A range of speed limit values for a train protection system, e.g., maximum speeds for different blocks, supervision and intervention speeds [Woodland, 2005], a brake release speed, a shunt speed, a non-supervised speed, a rollback speed, a reverse speed, an on-sight speed etc., will lead to a complex matrix of controls and rules which will lead to poor reliability and possible driver or maintainer confusion.

Basic speed limits will have to be imposed in the event of equipment failure but these should be limited in number and should cover reversing moves, shunting on ATP lines, coupling and uncoupling and recovery modes where possible.

Degraded Adhesion

All ATP systems fitted to trains operating over open areas of railway infrastructure will be subject to the effects of degraded adhesion from time to time. This is a particular problem in the temperate UK climate and has led to a number of attempts to mitigate the effects on train braking.

A number of strategies have been adopted, ranging from reliance on techniques such as "defensive driving", to sanding and to operating in protected manual mode on ATO metro lines. The Ebicab system allows drivers to input a "poor adhesion" rate when setting up the train for initialisation prior to entering service. Swiss railways use a special code of ETCS to achieve the same objective [Watson, 2003]. This issue must be addressed in respect of both technology and operating rules when introducing ATP.

Back-up Systems

The use of GSM-R radio for vital data transmission is still regarded throughout the railway industry as either "not fully proven" or "insufficiently reliable" for permanent, full-time operation. Railway administrations, both national and commercial, presently regard the use of radio transmission without a fixed block, visual line side signal back-up system as unacceptable from a reliability perspective. The questions raised about the reliability of GSM-R or GNSS technology and its availability might be answered by the use of two-tier systems or independent, parallel systems being used.

One of the main advantages of ETCS Levels 2/3 is that line-side signals can be removed and that a considerable rationalisation of line-side equipment can be achieved. This is expected to reduce installation and maintenance costs and to improve the life cycle costs. If back-up systems are retained, most of these benefits will be lost and costs will probably rise. This is likely to lead to failure to secure a business case for the viability of ETCS Levels 2/3 installations under existing cost/benefit analysis conditions. To overcome this problem, the reliability of the radio system must be improved and the methods for recovery from failures must be designed carefully to minimise loss of capacity.

In the meantime, experience with the Swiss trial of ETCS Level 2 shows that back-up line-side signalling system should be retained at least during the implementation and stabilisation phases of a project.

Future Strategy

High Speed Line Requirements

The difficulty of preventing driver perception overload has resulted in a situation in Britain where line-side signals are no longer acceptable for trains running at speeds greater than 125 mile/h. In order to provide an increase in operating speeds to 140mph and above, full ATP with cab signalling will be essential and the use of ETCS compliant equipment is the logical way to provide this.

The existing systems will have to be retained for conventional trains and may also be required for fall-back purposes at least during the early years of operation of any stand-alone ETCS Level 2/3 system, until reliability and operational experience allow removal of line-side signals.

Conventional Railways

It will be a logical progression for conventional railways to incorporate ATP using ETCS standards when signalling renewals become unavoidable. It is likely that this will be shown as a cost effective solution for renewals once the GSM-R network has been developed and the removal of full line-side signalling has been accepted as viable.

European Mandates

With the mandates issued by the European Community for the introduction of common train control systems on high-speed railways already being enshrined in national law and the forthcoming extension of this requirement to conventional railways, it will be necessary for UK main line railways to consider full ATP installation and, if it is decided to proceed, this will have to comply with the Technical Specifications for Interoperability (TSIs) issued by the European Community.

It is the authors’ view that the future strategy for train protection in the UK should be based on the continued use of AWS/TPWS until replaced by ATP in conformity with the ETCS requirements. The replacement will have to be carried out on a route upgrade basis.

It may be possible to consider other, low cost systems such as CBTC, as long as these are acceptable for lightly used community railways in remote areas with relatively low frequency services and a dedicated rolling stock fleet. However, it may be found that standardisation of ETCS across the whole network is more cost effective.

Life Cycle Approach

It is essential to view the implementation of any new system on the basis of a life cycle approach. The railway business does not allow for rapid technological change. The cost of equipment and the need to design to robust specifications to protect against a harsh operating environment leads to the need for a long period of depreciation before replacement. This restricts the capacity for adopting technological change and development. Shortages of skills will further limit change to what can be managed in a life-cycle replacement programme.

New electronics-based systems tend to have shorter life spans than traditional railway technology. Typical life periods of 15 years are now being adopted in project assessments for electronic systems as opposed to the traditional railway periods of 30-40 years. The 15 to 20 years may well be the maximum period for which suppliers will maintain spares for their products. However, well-designed, modular systems lend themselves to component replacement once subsystems become obsolescent, as has been shown in the case of the Victoria Line in London. A new ATP system will require a life cycle analysis to determine its cost-effectiveness.

A Business Case?

Making the case

In any strategic decision making process, the viability of the proposals from a cost/benefit perspective must be taken into account. In the case of the installation of full ATP on the UK railway network, there should be a review of the benefits and how much it will cost to achieve and maintain those benefits. Looking at these issues in depth and developing a strong business and safety rationale for implementation will lead to wider acceptance by railway companies and government agencies alike.

As an example, in examining the UK main line rail system, it can be seen that some 12% of the network has a line speed of 35mph or less and a further 55% has line speeds between 40 and 75mph. These lines are fully protected by TPWS. On the other hand, high-speed routes (only 17 per cent of the UK network) contribute 49 per cent of the ATP safety benefit and high-density passenger routes with line speeds of between 60 mph and 100 mph (22 per cent of the network) contribute 33 per cent of the ATP safety benefit. In conjunction with renewal requirements, it could be shown where the most benefit will be obtained from the introduction of ETCS compliant ATP.

With the completion of TPWS and TPWS+ over the UK network, the benefits of installing a fully ETCS compliant ATP system will be difficult to sustain in many areas.

A case for ETCS fitment might be augmented by operational advantages or the need for interoperability with the ETCS fitted network. It is possible that a detailed examination will show that a financial case for ETCS installation could be supported by the elimination of line-side visual signals and by improvements in line capacity. However, this will only be achieved when the technical reliability of the systems is universally accepted.

The question is further complicated when maintenance is considered. The goal of infrastructure managers is to reduce line-side or track-based hardware that requires regular maintenance. However, the balance must be found between the cost of supplying sophisticated technology and maintaining the software with highly qualified technicians on the one hand and the ground-based hardware requiring regular but cheaper maintenance on the other. With modern safe working practice requirements and the expansion of electronic signalling systems and its associated expertise, this balance will most likely favour ETCS systems.

In the case of new trains, however, the position over the provision of ETCS capability is clearer - all new stock is being provided with at least the physical capability of accommodating ETCS. It should also be a requirement that the future design of rolling stock should allow for the needs and sensitivities of the new generation of electronic control and protection systems.


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More articles on signalling:

The Development and Principles of UK Signalling - ATP Transmission and Moving Block Systems - ATO - Route Signalling - ATC - UK Warning & Protection Systems - UK Signalling - What the driver sees - Single Line Signalling - US Signalling - Docklands Light Railway Signalling