Train Maintenance


An essential ingredient in the successful running of a railway is a well maintained system.  Train maintenance is very important and this page outlines the methods and systems used in modern train maintenance.


Background - Maintenance Facilities - Access - Cleaning and Stabling - Toilets - Train Washing Machines - Wheel Lathe - Inspection Sheds - Shore Supplies - Lifting - Bogie Drops - Maintenance Workshops - Maintenance Programmes - Failures - Performance Measures - The Development of Train Maintenance


Railways are made up of complex mechanical and electrical systems and there are hundreds of thousands of moving parts.  If a railway service is to be reliable, the equipment must be kept in good working order and regular maintenance is the essential ingredient to achieve this.  A railway will not survive for long as a viable operation if it is allowed to deteriorate because of lack of maintenance.  Although maintenance is expensive, it will become more expensive to replace the failing equipment early in its life because maintenance has been neglected.

Rolling stock is the most maintenance intensive part of the railway system and is the most vulnerable if maintenance is neglected.  A stalled train will block a railway immediately and will reduce a timetable on an intensively used system to an unmanageable shambles for the remainder of the day.  Reliability is the key to successful railway operation and maintenance should be the number one priority to ensure reliability is on-going.

Maintenance Facilities

Trains require special facilities for storage and maintenance.  The basic design of these facilities as changed little in the last 100 or more years and, in many cases, the original sites and buildings are still in daily use.  Sometimes, these old layouts have made adapting to modern maintenance systems very difficult. 

The layout of a maintenance facility or depot will consist of a storage yard, a car cleaning area, an inspection and light maintenance shed, a heavy maintenance shop and, possibly, a separate locomotive shop or at least an area for locomotives if EMUs are the main service providers.  A typical facility with space for EMUs, works trains and locomotives might look like this:

Sample Depot-1.gif (34153 bytes)This diagram offers a layout for a depot (not based on anywhere in particular) but using best practice and with EMU operation in mind.  Doubtless people will have other ideas for improvements so any constructive contributions will be welcomed.  The operation of the facilities is described below.


An essential feature of any depot is good access, both road and rail.  Good rail access means that trains can get in and out of the depot without delaying trains on the main line and without upsetting operations within the depot.  It is no good if a train coming in has to stop at the depot entrance while the driver gets instructions from the shunter or depot control office and the rear of the train is still standing on the main line.  This can remove two or three paths from a timetable.  Usually a long access track into (and out of) a depot is required, if space is available.  If the railway is equipped with ATP (Automatic Train Protection), the changeover between ATP and manual operation will probably have to take place on this track.  This must be carefully incorporated into the depot track design.

Road access is equally important.  Large items of equipment may be needed to be delivered to the depot (transformers, pre-assembled traction units) and space to allow heavy trucks to get into the depot and turn, unload and exit must be provided.  In some cases, it is necessary to provide car delivery access by road.  Hard standing areas and unloading facilities like cranes or gantries must be considered when designing such a depot.  The hard standing needs to be located over or near a suitable track so that cars being delivered can be craned off the road vehicle and mounted onto their bogies, which have been delivered in advance and are already on the track.  The craneage can be hired in if the permanent installation of such equipment is not considered justifiable.

Cleaning and Stabling

Trains are stabled in depots or sidings when not in use and they need to be cleaned and serviced.  Cleaning means a regular exterior water wash and interior sweeping and dusting or vacuuming.  At longer intervals, seating upholstery and carpets must be shampooed.  Exterior washing is usually means a drive through washing machine which will wash the sides and, perhaps, the roof.   Suitable facilities must be provided in the stabling areas where trains are stored.  Water, power and toilet cleaning systems need to be provided in such areas, adjacent to each train to be serviced.  Access to trains must be designed so that cleaning staff can reach them safely whilst carrying their equipment.  This usually means floor height walkways alongside trains, or at least up to the first car of a set if through inter-car connections are available.

The layout of a stabling area is important.  Ideally, each road should have an exit route at each end, so that, if one end gets blocked for any reason, trains can still get out the other end.  There is no reason why two trains should not be stabled on each road if the length is right, again provided an exit is available at each end so that, if one train fails and is not sent out on time, the other is not blocked in.  Of course, site availability is always an issue and compromises are inevitable.  It may even be necessary to stable two trains on a single ended track.  Even this is viable if management of the fleet is flexible and allows trains due for entry into service to be swapped at short notice.  This is one of the essential skills of a good depot supervisor.

Train stabling areas are traditionally outdoors largely because of the expense of constructing large sheds.  However, covering the stabling areas with some sort of weather proof structure is always preferable.  It protects the trains and the staff working on or around them and reduces contamination by pollutants, frost, snow and wind damage.  A covered area will also provide some benefit in hot conditions and could help to reduce the air conditioning costs.


Modern trains which have toilets need to have them serviced regularly.  Although not shown in the above layout, a toilet discharge facility is required in any depot where trains have toilets.  The discharge has to be done away from the main buildings and where there is road access for the removal of effluent if it cannot be disposed of down a sewer.  Emptying of effluent tanks is normally followed by rinsing and then recharging of the system with flushing water containing formaldehyde to break up the waste matter.

Train Washing Machines

Train washer.jpg (69625 bytes) These (photo left) work on the same principle as a car wash, except that, usually, the train is driven through the wash and the washer itself stays in one place.  Some designs of train washer work like a very long car wash, where the train stands still and the washer moves during the cleaning cycle but these are rare.  Normally, water is used for a daily wash, while a chemical wash is used at less frequent intervals - usually several weeks.  Many daily washes have a detergent added to assist the process.  In referring to a daily wash, this might extend to three days between washes, depending on local practice and degree of pollution and dirt collection.

Washing machines require that the track on either side is straight for at least one car's length.  This is to ensure that the car goes into the wash straight.  There will also be a need for proper drainage facilities, complete with waste water management and, for the chemical wash, waste retrieval using a clarifier or separator.  It is usual to use recirculating systems nowadays, reusing the water from the final rinse at least, if not the ‘ready mixed’ water.

Washing machines may need a roof under certain conditions and they must be protected from adverse weather, particularly cold.  Freezing temperatures will play havoc with the pipes of a poorly protected machine.  Most operators do not wash under freezing weather conditions, so as to avoid ice forming around the doors and other moving parts.  Ice will quickly prevent train doors from operating and will render a train useless as a result.

Chemical washes are used for heavy cleaning and the chemicals used will often require the train to stand for some time while the chemical reacts with the dirt on the car body.  The standage must be protected against drips and the waste collected.  In places where there is space, it is advisable to do the chemical wash where it is protected from the weather.  Some form of special ventilation is likely to be required.  In some facilities, the chemical and water washes are contained in the same washing machine.

Wheel Lathe

Most modern depots are equipped with a wheel profiling facility known as a wheel lathe.  These are normally designed so that the wheels can be reprofiled while still on the train.  Removing the wheels requires the train to be lifted and this is an expensive business and very time-consuming.  To avoid this, the underfloor wheel lathe or "ground" wheel lathe was developed like the one shown in the photo left. 

Wheel lathe.jpg (105225 bytes) Wheels can be removed from a train by a "wheel drop", where the wheelset is lowered underneath the train into a basement below the depot floor.  Sometimes, whole toolrooms are provided in such areas but the ground conditions sometimes make such places difficult to keep dry and difficult to conform with modern evacuation requirements.

Modern wheel lathes can also reprofile a wheelset which has been removed from the train.  Otherwise a separate wheel turning facility has to be provided in the workshop.  Cutting has been the most common method of reprofiling but, recently milling machines, have been making a comeback as they can offer a longer tool life and better tolerance control on diameters.

Train wheels wear just as car tyres do and they need to be checked regularly.  When the wear reaches certain limits, the treads either have to be reprofiled to the correct shape or the wheels replaced.  Reprofiling wheels is a slow and expensive process but train and wheel design and maintenance has improved considerably over recent years, reducing the periods between visits for reprofiling.  Even so, there are still persistent cases of railways running into unforeseen or unusual wheel wear problems and the wheel/rail interface still needs a lot more research before it is fully understood.

Wheels on a bogie or wheels on a single vehicle must be reprofiled within limits compared with each other.  For example, a standard set for one type of passenger coach says that wheels in the same bogie must not vary in diameter by more than 5 mm.  Wheels under the same coach must not vary more than 10 mm on different bogies.  The most modern vehicles might require a tolerance as low as 3 mm.  When wheels which drive a speedometer are reprofiled, the speedometer will have to be adjusted to compensate for the difference in wheel diameter caused by the reprofiling.   

Some modern wheels lathes are designed to turn both wheelsets on a bogie at the same time.  These "double-headed" lathes have developed as a result of electronically controlled AC motors, which require that the motors in the same circuit turn at the same speed so as to match the inverter frequency.  This makes it essential that wheel diameters with motors within a traction power circuit are equal.

Although it might seem obvious, the roundness of wheels is important, especially at very high speeds. An eccentric wheel can cause extreme loads on the wheel, axle, bearing and suspension, leading to failures.  An "unround" (out of round) or eccentric wheel is alleged to have led to wheel tyre failure of a German ICE at Eschede in 1998, causing a high-speed crash with heavy loss of life.  The wheel is alleged to have had an eccentricity (difference between major and minor axes of the ellipse) of 1.1mm, against a limit of 0.6mm.   Wheels are often damaged by skidding during braking.  Skidding (called sliding) causes a flat patch (called a "flat") to wear on the tyre and, when the wheel begins rolling again after a slide, the familiar "tap, tap, tap..." of the flat will be heard.  Overheating during braking can also damage a wheel, as shown in the next photo:

Wheel Damage.jpg (54036 bytes) Even if wheels, by some lucky combination of circumstances, do not wear significantly, reprofiling to remove work-hardened metal is likely to be needed at around 1 million km, otherwise Martensite fragments can drop out of the wheel tread, leading to the type of damage shown in the photo above.  This damage can also be caused by local overheating during skidding and/or braking.

Leaves on the Line

One of the major sources of wheel damage in temperate climates with decidious trees is fallen leaves. 

Fallen leaves really can disrupt rail services – not just here in Britain, but all over Europe and North America. The scale of the leaf-fall problem and the cost of keeping services running smoothly is huge:

  • a mature lineside tree has between 10,000 and 50,000 leaves thousands of tonnes of leaves fall onto railway lines each year
  • there are 20,000 miles of track to keep clear in Britain
  • the annual cost of repairing damage to trains and track from leaf fall is over £10 million
  • lineside vegetation management costs over £5 million each year
  • the cost of felling large trees is between £20,000 and £50,000 per mile.

It is impossible to predict exactly when the leaf fall season will start and how long it will last, but the weather can provide a guide to its likely onset and how serious it is likely to be for the railway. A severe leaf fall season often follows a wet summer. It starts with a hard frost, followed by a high wind and a period of dry weather, which causes large amounts of leaves to fall over a short period of time. But traditionally, autumn is the season of mists and mellow weather, which spreads leaf fall over a longer period and reduces the severity of the problem. How do leaves on the line affect trains? Think of leaves on rails as black ice on roads and you’ll begin to understand the nature of the problem. We’re not talking about piles of dead leaves, but a hard slippery layer that coats the rails and is very difficult to remove.

Briefly, this is what happens: leaves are swept onto the track by the slipstream of passing trains light rain falls train wheels crush the wet leaves at a pressure of over 30 tonnes per square inch this compacts and carbonizes the leaves, forming a hard, Teflon-like coating on the rails. As a result, trains have to operate at slower speeds to ensure safety and to reduce the potential for wheel slip and spin. This means that drivers have to brake earlier for stations and signals and move off again more slowly. Consequently, train services can be delayed. If a train can'’t move because its wheels can’t grip the rails, often there is no alternative route, therefore following trains are delayed or have to be cancelled.

In addition to causing severe disruption to passengers, the damage inflicted on train wheels during sliding and spinning on rails is considerable and means some trains have to be taken out of service for expensive repair. The rails too can be damaged, costing many thousands of pounds to repair each year. So what is the rail industry doing about it?

Network Rail, the UK body responsible for maintaining the rail network, is working to eliminate or minimise the problem of leaves on the line. They has a fleet of special 'sandite' trains, which spread a gritty paste on the rails to give trains improved adhesion. Known problem areas such as deep cuttings and steep inclines are targeted in order to minimise delays. There are also static machines to apply sandite at known trouble spots and mobile applicators, which can be used by track workers.

High pressure water jets are also used to remove crushed leaves before they form a hard coating. Leaf guards can be positioned around the track to stop the leaves being blown onto the rails and, in some cases, it is necessary to fell problem trees. However, to protect the environment, these are replaced with smaller leafed trees such as hazel, cherry and holly. Network Rail’'s tree surgeons take advice from conservation specialists to minimise the impact tree management can have on wildlife. For example, no work is planned during the main nesting season. Trains are also fitted with sophisticated sanding equipment to improve traction on slippery rails,– the equivalent of ABS on a car. The driver can apply the sand when wheel spin occurs during acceleration, or it can be applied automatically (Source Network Rail).

Inspection Sheds

Special facilities are required to carry out rolling stock inspections.  A properly constructed building, capable of accommodating a whole train, should be provided.  Access to the underneath of the train is essential and this must be designed to allow reasonable working conditions and safety.  There are various ways of doing this.  The most common used to be a pit provided between the rails of the maintenance tracks and, sometimes, pits on either side of the track as well, to allow access to the sides of the underframe equipment.  A more common approach today is the "swimming pool" design, where the floor of the shed is sunk and the tracks are mounted on posts.  This gives better access and improves the light levels under the cars.

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Inspection shed with pits

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Inspection shed with "swimming pool".

Shore Supplies

Inside train sheds and shops, it is necessary to provide shore supplies for trains and power for tools and maintenance equipment.  Where overhead electric traction is used, the overhead wires are usually installed inside inspection sheds but not in shops were vehicles are lifted.  If it is necessary to get access to the roofs of trains, the overhead current must be switched off and the switch secured by a lock.  Any person working on the roof will have a personal access key for the lock to ensure the current remains off until the work is complete and it is safe for it to be restored.  The access stairs to the roof level walkway will also have a locked gate which can only be unlocked if current is off.  A system like this is shown in the first photo below.

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Interior of inspection shed showing overhead walkway with access gate.

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Knife switch and shore supply lead on a subway car.

In the modern depots built for the Eurostar Channel Tunnel trains in London and Paris, the overhead catenary in the workshops is designed to swing away from the roof when required to allow access to the roof mounted equipment.

The second photo above shows a shore supply attached to a New York City subway car, which has a third rail traction system.  For safety reasons, the sheds are not equipped with the third rail, so a supply through a long lead is provided.  The lead is plugged into a socket on the side of the train.  Various systems are used around the world.  In the case shown above, the shore supply socket on the car has a knife switch to isolate the current collector shoes from the supply.  This is to avoid electric shock risks to persons working on or near the shoes.  In older installations, this facility is not provided and a shore supply will energise all electrical equipment on the train, including the shoes.  In these situations, special precautions have to be taken to ensure no "overhead leads" or "stingers" are inserted on a train being worked on. 

Star Overhead lead.JPG (46055 bytes)For third rail systems, the shore supply cables are usually fed from electrified rails suspended from the shed roof.  The cables are hung from trolleys running along the rails so that the supply is available along the whole track. 

It is common to use the overhead leads to power the train out of the shed until the leading collector shoes are in contact with the current rails outside the shed.  This is sometimes called "railing".  The train is then stopped and the overhead leads removed.  The leading car is then used to drag the rest of the train out of the shed.  Care has to be taken to ensure all leads are removed before allowing a train to leave the shed and enter service.

In the US, the "railing" procedure is often performed "on the fly" (with the train moving), since the shore supply is connected directly to the collector shoes, which are large paddles.  The live end of the "stinger" rests in a hole on the shoe or is clipped to the shoe by a large spring clip. 


The traditional method for accessing bogies was to lift the car body off the bogies by use of an overhead crane or cranes as shown in the left hand photo below.  Each vehicle to be lifted has to be separated from its fellows in the train and dealt with separately.  If one car in a set is defective, it has to be uncoupled and pushed into the shop for lifting.  To access the bogies, the overhead crane is used to lift one end while the bogie is rolled clear and then the body is lower onto stands.  Then the other end is lifted, the bogie rolled clear and the body lowered onto two more stands.  Motors, wheels and other items can then be worked on or removed from the bogie as necessary.  Naturally, this takes up a lot of track space in the shop and requires time spent on separating the vehicle from the train and then from its bogies.  For overhauls, the bogie may be removed to a special area where it is placed on stands for stripping and refitting work.

A quicker lifting method is to use two cranes which lift both ends of the car body together and free both bogies at the same time.  The body can then be removed to another part of the workshop for maintenance.

NS Car lift.jpg (96202 bytes)
Car being lifted in a workshop by a pair of overhead cranes
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Jacks are the usual method of lifting nowadays.  Vehicles can be lifted individually or, if a fixed formation is used for normal service, more recent practice has been to lift the whole train set.  This is done by synchronised jacks.  The jacks are linked by control cables and controlled by one person from a control desk.  The big advantage of this system is that you don't have to break up the train into individual cars to do the work on one vehicle.  The time saved reduces the period the train is out of service.

Underfloor jacks are also becoming more popular as shown below.

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Workshop with underfloor jacks
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Car lifted with underfloor jacks.

Again, the lifting system is synchronised to allow several cars to be lifted at the same time if necessary.  This is quicker then uncoupling each vehicle, especially if there is only one requiring attention.

Rolling stock can be lifted on a track where there is no pit, especially if there is a need to exchange a piece of underfloor equipment.  A fork lift truck can be used to do this if there is enough room at the sides of the trains for it to manoeuvre.  Otherwise a small scissors lift table can be used.  In all cases, it is essential to ensure that the floor will take the weight of the train raised on jacks.  Most modern rolling stock is designed to be lifted with its bogies still attached so that exchange of one piece of underfloor equipment can be carried out on a lifted train without disturbing any other cars. 
Bogie Drops

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Bogie drop table
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Bogie being lowered

Another system used in some shops is the bogie drop.  The train is run over the lifting road, which has a pit and is positioned so that the bogie to be removed is located over a special section of track.  The bogie requiring removal is disconnected from the train, using the pit for access.  The car bodies are then lifted, leaving the disconnected bogie on the track.  The section of track where the bogie is located can now be lowered into a basement area and the bogie removed and replaced by a fresh one.

A variation of this system has the train lifted by raising the sections of track under the bogies.  The car bodies are then supported by stands placed under them and the bogies to be changed are disconnected.  Once free, they are lowered to floor level and serviced or exchanged for new bogies.  Turntables can be installed to assist in the removal of the bogies to other maintenance areas.

Maintenance Workshops

It is still common to see workshops for railways provided with tooling and equipment to allow a full range of engineering tasks to be undertaken.  This will include milling, boring, grinding, planing and cutting machines as well as part cleaning facilities (including bogie washing and car underframe cleaning or "blow-out" as it is sometimes called), plus electronic and pneumatic testing shops.  Good storage and materials management facilities are also needed.  Computerised systems are now widely available. 

Not only does the rolling stock require maintenance but also trackwork, traction power equipment, signalling, communications equipment, fare collection systems, electronics of all types and buildings maintenance.  The main depot of a railway has to be equipped to handle all these.  Works trains will be needed to ferry equipment and staff to work sites along the line and these will be serviced at the depot.  Refuelling facilities will be needed for diesel locomotives and DMUs.  Storage for hazardous materials and fuel must be in a secure place with proper fire protection facilities.  Waste disposal must also be properly managed and waste recovered if possible.

Maintenance Programmes

Rolling stock maintenance can be programmed in one of three ways; by mileage, by time or by conditioning monitoring.  Of these three methods, condition monitoring is the most recent.  Traditionally, maintenance was carried out on a time basis, usually related to safety items like braking and wheel condition.  Many administrations later adopted a mileage based maintenance system, although this is more difficult to operate as you have to keep records of all vehicle mileages and this is time consuming unless you have a modern train control and data gathering system.  There is also the fact that a train will deteriorate just as quickly if it is stored unused somewhere as it would if it was being run in service every day.  Only the items which deteriorate will vary.

Modern trains should be able to run for some weeks without a maintenance inspection.  One train operating company in the UK wants to get inspection intervals out to 90 days on its new EMUs.  Comparing this with the 3 days between inspections which electric trains got at the beginning of the 20th century and the 7-daily inspections still being carried out in the 1980s on some similar fleets, shows the rapid progress of the last few years.  It is impossible to give fixed time or mileage periods here for maintenance as each type of train varies.  There are often special rules for high speed trains and for heavy freight.  Individual railways have adapted their maintenance to the local conditions and, in many places, certain types of safety inspections are required by law.  As an example, the Channel Tunnel Shuttle trains cover about 5,000 kms a week and get an initial inspection every seven days.  However, the French high speed trains (TGV) are given a daily visual inspection of the underneath and the pantographs.  The toilet system is emptied every three days and the trains return to their base depot every 5-6 days for their 4,500 kms inspection.   Examination of equipment such as traction motors and bogies takes place every 18 days.

Condition monitoring is achieved by checking the operation of the equipment and only changing something if it shows signs of wear beyond preset limits.  The checking is often done using on-board monitoring and storing the data gathered in a computer for downloading at the maintenance facility.  Of course, it is a recent development made available by the introduction of information technology on trains.  Such systems are now becoming so sophisticated that it is possible to have failure predictions of some items of equipment.  A combination of on-board data gathering and depot maintenance systems have been developed into complete maintenance management systems on lines where modern rolling stock has been introduced.


As already mentioned, reliability is the key to running a successful railway.  If the equipment, especially the rolling stock, is not reliable, the railway is not workable.  A good railway management will keep track of its performance and its failures and, by this means, ensure that problems are eliminated before they become endemic. 

A number of methods can be used to monitor performance.  The traditional way was to measure on-time performance.  The number of minutes late of each delayed train was recorded and collated into daily, weekly, monthly and annual statistics.  Usually, the time of arrival at the destination station was the basis but an intermediate delay is often also used to quantify a delay to a service.  The cause of each delay is decided - yes, it does require a decision, as we will see later - and the cause investigated.  In the case of rolling stock, there is probably a technical reason for the delay and there is usually a check to see if other, similar incidents have taken place.  If so, there may be a design fault which needs a modification to the fleet to rectify.  Checks are also done on maintenance procedures to ensure that the process is being carried out properly and, if so, whether the system needs to be modified.

Causes of delays are investigated to find out what happened.  Imagine the case of a train which comes to a halt in the middle of nowhere.  The driver notices that he has lost all the brake pipe air so the emergency brake applied.  After finding that the conductor has not stopped the train and no passenger alarm valves have been operated, he starts trying to find out the cause.  After a while he find the cause - a burst hose between coaches.  Back to get a spare hose.  Oh dear, he hasn't got a spare hose.  He has to isolate the defective portion of the train and limp on to the next  station where he can get a hose and repair the train.  Delay to his train?  25 minutes actual delay at site plus 17 minutes lost going to next station plus 35 minutes replacing the hose.  Total delay 77 minutes.  But what was the cause?

Initial observation of this incident would suggest a defective hose.  The delay would be allocated to rolling stock and the engineering department would have a few words with the supplier to tell him what happened and discuss possible causes and remedies.  However, in our case, an enquiry is held because the conductor on the train says he heard a loud bang under the train just before the emergency stop.  The enquiry finds that, upon investigation, a shovel was found by the track in a bent and battered condition.  The underneath of the train shows signs that it was hit by something.  The damaged hose shows cut marks near the burst.  It is concluded that the permanent way workers in the area had left a shovel on the track, it struck the train and damaged the hose.  Allocation of delay: "Permanent Way" department.  It wasn't a rolling stock failure at all, although it was aggravated by the lack of a spare hose, which is a rolling stock error.  Eventually, as a result of some bargaining, the delay is split between the two departments, both of whose performance is measured on train service reliability.

Such investigations and the subsequent bargaining were traditionally part of the railway culture and have recently become more important since the commercialisation of the business.  No one wants to be blamed for delays, since service performance is part of their contract and part of their payment structure.

Performance Measures

Rolling stock performance in respect of failures can be measured by MTBF (Mean Time Between Failures) or MDBF (Mean Distance Between Failures).  It is sometimes measured by numbers of failures per year, month or week but this may not represent an accurate rate consistent with mileage.  On the other hand,  rolling stock does deteriorate rapidly in storage and this, in itself, produces failures, although these may not be the same failures seen under normal service conditions.  Failure rates are sometimes quantified in service performance by availability.  The performance is expressed as, for example, 95% availability.  In other cases it is quantified as, say, 92% on-time.  This is more unreliable as a statistic if the on-time regime is cushioned by huge amounts of "recovery time", as is often the case today. 

Performance monitoring also depends on the real definition of a delay.  At one time, the Inter City services in the UK were using 10 minutes as the definition of a delay.  This was much derided in Europe, where on-time performance meant just that.  If you were not on time, you were late.  Perhaps a more equitable way to define a delay is by the loss of a train path.  Most main lines will give a three minute headway or 20 trains per hour, assuming equal speeds and performance.  A three minute delay will therefore lose a path and, in the commercial structure of a modern railway, deprive the track owner of the sale of a path to another train operating company.  In a metro or suburban operation, the path will be two minutes or slightly less, so a two minute delay would be an appropriate measure of performance.

One other point about performance is that time out of service is as important as the frequency or duration of failures.  Another measure applied to equipment is the MTTR (Mean Time To Repair).  A short delay which requires a train to be taken out of service for repair become more critical is the train takes a week to get back into service.  It's not good design if the train owner has to lift the car off its bogies in order to replace a fuse.  Short MTTR is another important part of good rolling stock performance.

The Development of Train Maintenance

Leaving aside the question of steam locomotive maintenance, which is another subject entirely, the regular inspection of motive power, coaches and wagons has long been a part of the railway culture.  The need for inspections was based on the need to ensure the good condition of the structure of the largely wooden bodies of the coaches and wagons and the integrity of the wheels, axles and braking systems. 

Example - Wheel Inspection

As an example of how train maintenance has developed, we can look at wheels and axles.  Wheels and axles were vulnerable to fracture, particularly in the early development of railways when manufacturing techniques were not as sophisticated as today, and they were checked daily for visual signs of damage.  Many railways painted a white mark over the wheel tyre and hub so that any movement of the tyre on the hub was immediately noticeable.  Wheels were also "tapped" - struck with a hammer to ensure a "ring" was heard so as to confirm there were no cracks.  In spite of these checks, there were occasional and sometimes spectacular accidents due to wheel or axle fractures on trains in service.

As early as the 1930s, techniques were developed to test axle integrity by electrical means.  Magnetic particle testing was one system used, where energised particles of steel were applied to axles to determine the location of cracks.  In the 1950s, an early form of ultrasonic testing was used.  Nowadays, such systems are standard.  This type of development process has taken place for all rolling stock systems, including those on locomotives, coaches and wagons.

Example - Brake Maintenance

Train brakes were always inspected regularly.  In the early 1900s, electric multiple units (EMUs) used in London (UK) were inspected every three days, most of the work concentrating on replacing brake shoes (often called brake "blocks" in the UK) and adjusting the complex rigging used to connect the single brake cylinder to the blocks on each of the wheels.  Many designs used two blocks on each wheel, a design known as "clasp" brakes.  Setting these up to get even braking on all wheels was a skilled job requiring at least two men under the vehicle.  As the blocks were usually made of cast iron and weighing around 28 lbs. (12.7 kilos), they were awkward to access and fit.  On cars with traction motors, it was more difficult due to the reduced clearances.  Later designs reduced the rigging, so that each bogie had a cylinder or, on modern trains, each wheel or even each block had its own brake cylinder.   The skill requirements were reduced but the bogie became very crowded and block changing became a job for very slim athletes.

Modern trains usually have disc brakes and pads are used instead of blocks.  These are easier to change, being smaller than blocks, but the modern bogie is still a very crowded place - see Bogie Parts.  During inspections of brake parts, only a percentage of blocks or pads get changed and a lot of time can be wasted looking at them to see the state of wear.  Recently, a system developed in the UK called Padview, has been introduced to some depots.  Padview takes a video image of the brake pads of a train as it enters the depot and compares the image with a computer stored image of a pad.  The comparison generates a "go-nogo" output to tell the supervisor if the pad needs changing.  A similar system has also been developed for checking wheel wear.

Aside from the mechanical features of brakes under the cars, a test of the operation of the braking system has traditionally been mandatory on a daily basis.  Not all railways insist that wheels are checked to see if the blocks are applied on each but this is normally done during "reblocking".  However, a test to see that the brake pipe is continuous throughout the train is essential, especially after coupling or uncoupling and after maintenance work.  This is normally done by charging the brake pipe fully from one end of the train and checking that it has charged at the other end.  This is followed by a reduction in pressure from one end and a check that the pressure has fallen at the other end.  On US freight railroads, the train will have a system which electronically checks the pressure at the remote end to eliminate the manual check.  See EOT Device for more information. 

Modern brake systems can incorporate pressure sensors which determine brake cylinder pressure or even brake pad pressure.  The reading is sent to an on-board computer or data unit and alarms are provided in the driver's cab if a unit reveals an unsafe condition.  In maintenance, this facility can be used to download data indicating wear, failures or likely failures.  Such facilities are now part of modern train maintenance management.