US Railroad Signalling


On railways all over the world, signals are used to indicate to the driver (engineer in the US) of a train how they should proceed. The way this is done in the US (and most of Europe except Spain and Norway) is quite different compared with the UK. In the UK (and other countries using UK based systems), signals are designed to show the driver the state of the road ahead. For example, a signal will show that the line is clear ahead and will also say how far ahead it is clear. The driver, using his knowledge of the line and of the train he is controlling, will make a judgement about how fast he can safely let his train go and will proceed accordingly. In the US, signals show engineers the speeds they are allowed to go. This is known as speed control signalling. They do not actually need to know how far ahead the line is clear. The speed they are allowed to do will depend on the type of train the driver is controlling.

ABS and Interlockings

In the US, like the UK, signals are classified into two general types. In the UK they are referred to as automatic and controlled, in the US they are known as Automatic Block Signals (ABS) and Interlocking Signals respectively. The two classifications are similar in that automatic signals work without manual controls while controlled or interlocking signals usually cover junction areas and require some form of additional controls operated by a signal tower (signal box in UK) or control room.

Overlap or Safety Block

Another feature of US railroad signals is that they do not have the 200 yard overlaps that are normal in the UK. The usual method of providing a safety margin beyond a stop signal in the US is to allocate the whole of the next block as the overlap. This is similar to the principle adopted on metros which use ATP.

Bi-Directional Signalling

Many US lines are equipped with full bi-directional signalling.  You will often see a block boundary with two signals, one facing in each direction.  The signalling operates exactly the same, regardless of the direction of running.


Any operator in the US who wants to run trains over 79 mi/hr has to have some sort of automatic train stop (ATS), automatic train control (ATC) or cab signalling system (CSS). These names all mean that the driver gets some type of in-cab indication and a warning of signal conditions. There are basically two systems; those which provide a warning like the UK AWS and those which regulate speed, like an ATP system.

Dark Territory

In the US, there are still large sections of lines which have no signals. This is almost unheard of in Europe because train traffic is normally a lot more dense. In the US, the unsignalled lines are usually long, single line sections in remote areas and there are thousands of miles of them. They are commonly referred to as dark territory. Trains are permitted to pass from one area to another by the use of train orders or track warrants, nowadays transmitted by radio between dispatchers and train crews.  Passing loops, called sidings in the US, usually form the boundaries between areas. There are elaborate rules for ensuring safety and accidents are rare.

Single lines with Signals

Some single line sections in the US are equipped with ABS (automatic block signals) to allow two or more trains to follow each other closely along the single line between sidings. The signals are provided for both directions as shown in the diagram below.  Often, the entrances to sidings are not controlled by interlocking signals and turnouts for sidings (passing loops) are hand operated.

Figure 1: Schematic of single line with US type bi-directional AB signalling. A westbound train is approaching the signle line and all the WB signals are showing green. The last two EB signals show that the track ahead of them is occupied. Even though there are junctions at the end of the signle line, there are no interlocking signals protecting them. The turnouts are handworked by the train crew.  Diagram: Author.

There are no signals protecting the entrance to the signal line. The crew will be given authority by Track Warrant over the radio to enter the single line and then they will observe signals. The reason for this type of operation is to allow more than one train to proceed along the single line. Without the signals, successive trains have to follow by time interval, a rule still used in North America but which has not been allowed in the UK for over 100 years.

The next diagram shows the sequence of signals as two westbound (WB) trains pass through the single line section.  The signal indications are similar in meaning to the British "stop, caution or distant and clear" indications.


Figure 2: Schematic of single line with US type bi-directional AB signalling. Two westbound trains are passing through the single line. Note the sequence of signals in the WB direction. All the signals for the EB direction have been switched to red.  Diagram: Author.

On single lines in the US, it is not unusual for sidings to be equipped with hand operated turnouts (points), even if the line is equipped with automatic signals.  This means that the driver of a train approaching a siding has to stop his train, even if the signal is showing a "proceed" aspect, and operate the turnout by hand to enter the siding.

Some sidings have spring loaded turnouts which are set to ensure that trains from opposite directions enter a different track without the crew having to stop and set the turnout by hand.

Operating Philosophy

In the UK, trains have been regulated by fixed signals since shortly after railways were first opened in the early 19th century. In the US, signals were the exception rather than the rule and many railways' rulebooks reflect this in their treatment of the rules.  Signals were (and still are in some places) regarded as an adjunct to the railway rather then part of it. With this in mind, we can now look at US signalling in general.

US Signal Layouts

A train passing along a signalled route will see an arrangement of signals which will appear somewhat as described in the following paragraphs.  Every 2 miles or so the train will pass an Automatic Block Signal (ABS).  All ABS signals for all tracks and both directions are located right next to the block entrance at the insulated rail joint (IRJ).  So, as the engineer (driver) passes from block to block its almost like passing through a pane of glass.  This is enhanced by the fact that for the most part there isn't much along the wayside but at a block limit the engineer will see signals, relay boxes and rail joints. 

OK, the engineer is passing block after block and all the signals have one head but then he gets to a block limit where the signal in his direction has two heads. These are the distant signals for an interlocking. They are still automatic and still retain a number plate that identifies them as automatic. Interlocking signals do not have number plates. After proceeding a little farther down the line the engineer reaches the interlocking entrance.  Across all tracks, in his direction only, is a line of signals. These signals have two or three heads and no number plate, which identifies them as absolute (stop) signals. If there is a proceed indication, the train crosses the boundary defined by the IRJ and signals and enters the interlocking. The train then passes over the points, it passes the signal tower or relay shed, perhaps passing over some more points and then reaches the "exit" signals. The exit signals actually have no bearing on this train whatsoever because they are all facing the opposite direction for incoming trains. 

The entrance signals not only govern the interlocking, but also the next block. However the exit signals and associated IRJ define another boundary and, after the train completely passes this boundary, is it out of the interlocking, free of dispatcher control and under automatic signal rules. Note that, while any part of the train is between the Home (entrance and exit) signals, it is working under interlocking rules. Once it crosses the boundary defined by the Home signals it is under Rule 251 or 261 operation.

The train continues on to complete the same steps through each interlocking. Every so often a train will pass from one interlocking right into another (some interlockings also have sub interlockings that are completely independent). The first ABS limit the train reaches will be the distant for operation in the other direction.


In the US, trains are given what is referred to as "precedence". This means that each type of train has a "pecking order" in terms of priority of movement. Precedence is determined first by the timetable, then by the type of train and then by direction.  Different railroads have their own precedence rules but the principles are the same.

Train Orders and Track Warrants

The foundation of US railroad signalling philosophy is single line operation without fixed signals. Signalling was only introduced for sections of line which had too many trains to handle under manual rules or where there were junctions. Trains are handled by Train Orders or Track Warrants. An explanation of the train order and track warrant process is available on the single line operation page.

Signalling Commands

The US Automatic Block Signal (ABS), i.e. one without any manual control and operated by trains passing through track circuits, shows four basic commands. The way the signal displays the commands, in other words the aspect, varies from railroad to railroad and often from division to division in a railroad. There are also variations in the meanings of signals which appear to look the same. The basic commands, however, are:

Stop, Approach, Approach Limited and Clear.

The US has the "stop and proceed" signal system seen in the UK but it is referred to as a "permissive" signal. The driver is told, "You are allowed to pass this signal after stopping but you must proceed at a speed which allows you to stop your train in half the available sighting distance." There are some stop signals at interlockings (therefore they are not ABS) where it is forbidden to pass and these are called "absolute stop" signals. They invariably show a different display to the permissive stop signal and it normally includes two red lights.

As US signals are speed limiting, a signal displaying "Approach" means the equivalent of the UK single yellow - "be prepared to stop at the next signal" but, additionally, the US rule says, "also keep your train speed down to less than 30 mi/h (often less for freight)". "Approach limited" (UK = double yellow) would mean "you should be doing 30 by the time you get to the next signal but not more than 45 mi/h now".

In the US also, there are four common terms used to instruct crews about permitted train speeds as follows:

Slow = 15mi/h

Medium = 30 mi/h

Limited = 45 mi/h

Restricted = 15mi/h within an interlocking or 20 mi/h outside it or the speed which allows you to stop within half sighting distance. It is the speed you are allowed to do if you have passed a red permissive signal.

Some of the speeds may be lower for freight trains. 

The fundamental purpose for US signal indications is to inform the engineer of the approaching train of the speed rule that he must apply to his train’s movement. There is a list of the signalling rules applied to most railroads in North America at NORAC Signal Aspects.  It shows each signal display and the rules appertaining to that display. Most rules have a number of different signals according to the original railroad company rules, the highest range being fifteen. 

Interlocking Signals

Interlocking signals in the US represent the UK "controlled" signal; i.e. one that is controlled from a signal tower (cabin in UK) or any sort of control room. Interlocking signals offer a great variety of signal displays and commands which can be confusing. In addition to signals showing what speed you are allowed to do because of the route which is set for you, there are some which indicate a speed restriction "within interlocking limits”. The term interlocking is now creeping into UK parlance.

A sample series of commands looks like this:  (Note that the commands are all speed related and they cover AB signals as well.)

Clear - Proceed at normal speed for your train, you will get the straight route ahead.

Approach - Be prepared to stop at the next signal and reduce train speed to 30 mi/h

Approach Slow - Be prepared to stop at the next signal but approach it at slow speed (15 mi/h); usually because of passing through a crossover.

Advance Approach Medium - Proceed but approach second signal ahead at medium speed (30 mi/h)

Approach Medium - Proceed but approach next signal ahead at medium speed (30 mi/h)

Approach Limited - Proceed but approach next signal ahead at limited speed (45 mi/h)

Limited Clear - Proceed but at limited speed (45 mi/h) within interlocking limits

Medium Clear - Proceed but at medium speed (30 mi/h) within interlocking limits

Slow Clear - Proceed but at slow speed (15 mi/h) within interlocking limits

There are some variations for different railroads but the range of speeds is similar.


It is worth adding a few notes here about the use of the word "approach" and how it is applied to US signals. Approach is the US term which would be known as "caution" in the UK. Approach Medium, Approach Limited and Advance Approach are all commonly used to describe certain types of signal commands in the US. There are basically three types of Approach signals: "Approach ...........", "............ Approach" and just plain "Approach". 

Approach tells you how fast you have to be going by the next signal only. Except in the case of Approach Slow, they say nothing about how fast you can go before the next signal. Also an "Approach ..........." signal informs the driver that the next signal is not at Stop but also not at full Clear. For the interests of safety, trains with ATC are limited to 45 mph or less after passing any type of "Approach ..........." signal but, going by the letter of the rules, the driver must only be doing the proscribed speed by the next signal. Furthermore, while trains with ATC are limited to 45 mph after passing both an Approach Medium or an Approach Limited, trains which pass an Approach Medium must slow to 30 mph by the end of the block, while trains which pass an Approach Limited can continue to travel at speeds up to 45 mph.

A "............ Approach" signal tells the engineer to reduce speed at once and then to expect a stop signal. In the case of Medium Approach the driver must begin to reduce to medium speed as soon as the Medium Approach signal becomes visible. This also serves as an informal overlap.

A simple Approach signal tells the driver to expect stop in one or two blocks and proscribes an immediate speed reduction.  Advance Approach and Approach Medium or Limited are not always used interchangeably. Approach M/L makes use of signals with two or more "heads" but if you want to install three-block protection then it's easier to use Advance Approach. 

Advance Approach proscribes a speed limit of 45 mph while Approach M/L doesn't. Approach M/L implies non-stopping signals ahead while Advance Approach implies the second signal is red.  In the US, with huge freight trains the engineers really need plenty of time to prepare to stop and therefore they need signals that give them warning of a stop and others that warn them of a required slow-down. 

"Approach ..........." signals are most often used to indicate that a train will be taking some form of diverging route at an interlocking. Advance Approach signals warn of a stop in two blocks, Approach warns of a ABS (permissive) stop at the next signal and Medium Approach warns of an absolute stop at the next signal.

Types of Signals

As we have seen, there is a wide variety of signals in the US. Each R.R. originally had its own system but now, due to mergers, take-overs and split-ups, you can expect to find a mix of signals on any system. A good explanation of the different types is provided by Douglas A. Kerr in his site Rail Signal Aspects and Indications.

The Imposition of ATS/ATC

The US uses automatic train stop (ATS) or automatic train control (ATC) on busy lines or where higher speeds are required.  ATS is similar to the UK AWS, while ATC is a form of automatic train protection (ATP), as it is called in the UK.  In 1922, the US Interstate Commerce Commission (ICC) told railroads that it wanted them to install some sort of ATS or ATC on hi-speed lines as a safety precaution. The ICC made no regulations at first but it warned that it would do so in the future. Initially, several companies began to build ATS/ATC systems but then the depression of the 1930s, followed by WW2, slowed development. In 1951, the ICC made good on its word and mandated a nationwide 79 mi/h speed limit on any track not equipped with some sort of ATC/ATS. By this time many Americans had bought cars and given up on train travel so a number of railroad companies felt that ATS/ATC was not worth it and just accepted the speed limit but there were a few notable exceptions. These included the Pennsylvania R.R., which was a firm believer in safety systems.

The 79 mi/h speed limit is still in effect, although on some lines in the western US Amtrak has received permission to go up to 90 mi/h.  On the east coast it is illegal for any non-CSS (Cab Signalling System) equipped train to run on CSS territory. 

Types of ATS/ATC

ATS operates from track mounted inductors.  At the first restrictive signal, the inductor acts to operate a warning noise for the driver who has a few seconds to acknowledge it and start braking the train or there is an automatic brake application.  The system is sometimes used with cab signalling (called CSS), where the signal displays are shown in the cab.  This requires a continuous track to train transmission system.  On some lines equipped with cab signalling, there are no wayside signals.

ATC requires continuous track to train transmission since the speed of the train is being constantly monitored and cab signal displayed to the driver.  If speed limits required by the signal displays are not adhered to, the ATC system will apply the brakes.  ATC also operates over lines which are not equipped with wayside signals.

Here's how the system works:  As a train passes signal A at the start of the block, a CSS coder at the end of the block sends the CSS code into a rail.   The code consists of pulses of 100 Hz AC, 180 pulses for Clear, 120 for Approach Limited, 75 for Approach and 0 for Restricting or Stop. 

The largest user of ATC with cab signal and with wayside signals was the Pennsylvania R.R. (PRR).  The PRR had always been a leader in safety and was one of the first RRs with air brakes and knuckle couplers as standard.  In the 1920's the PRR was busy electrifying and replacing old semaphore signals with yellow position light signals.  They listened to the ICC and decided to install a Cab Signalling System on all main routes.  This grew to include over 1100 route and 3000 track miles.


Mike Brotzman, and "The Railroad, What it is, What it Does" by John H Armstrong, 1993, Simmons Boardman Books Inc.

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