North American Signaling:

SP single track Automatic Block Signaling

by Carsten S. Lundsten. Updated September 25, 2000

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An SP Single Track ABS Line
Signal Aspects and Indications
SP Single Track ABS Protection of Trains
Protection of Opposing Trains between Sidings
Protection of Opposing Trains when Approaching Sidings
Protection of Following Trains
An SP Single Track ABS Example
Increasing Train Size and Speed

This document explains the Automatic Block Signaling (ABS) system used on parts of the former Southern Pacific single track lines. ABS systems for single track were designed in the timetable and train order days to allow trains to safely follow each other closer than what would have been possible with timetable and train orders alone. Timetable and train orders still provided the safety for opposing trains (i.e. set up meets) and was the means of dispatching. Thus the requirements for the ABS system to provide safety for opposing trains were not very strict. Most single track ABS systems provide a nearly complete protection against opposing trains - but only nearly complete. The ABS is able to prevent most head-on collisions but a few situations may result in a head-on collision at low speed.

The SP single track ABS described in this document provides a nearly complete protection against head-on collisions. Actually the safety structure of the SP ABS is identical to the Absolute Permissive Block signaling systems, except for all signals being permissive. The lack of absolute headblock signals allows a train to proceed onto a line with an opposing train, though at restricted speed. Unless timing is very unfortunate signals will indicate to the opposing train that another train is on the line and thus also have this train at low speed.

Trains were formerly dispatched through timetable and train orders, nowadays as part of the Direct Traffic Control system used by the former Southern Pacific.

An SP Single Track ABS Line

The ABS system protects a single track line including any sidings along it. The sidings are used to meet or overtake trains. All signals are automatic and there is no interlocking or CTC control of an ABS line. Switches are thrown by the train crew as needed. ABS lines may span hundreds of miles without any controlled signals. The SP ABS systems originally used lower quadrant semaphore signals. Almost all of these signals are gone today, replaces by color light signals or the line has been converted to CTC. This document illustrated both semaphore and color light signals, as the latter are easier to read on the somewhat compressed drawings that follow. In the semaphore examples both signal arms and lights are shown to give an idea of day and night signal aspects. Some signals that are not important for the example are shown shaded.

A typical SP single track ABS line could look like this, shown in its normal state without any trains present and all signals showing "Clear":

Signal Aspects and Indications

The following signal aspects and indications are used on these lines:
Signal Aspect  SP Rule  Name  Indication 
9.1.3 Clear  Proceed 
9.1.7 Approach  Proceed prepared to stop at next signal, trains exceeding 40 MPH immediately reduce to that speed
9.1.13 Stop and Proceed  Stop, then proceed at restricted speed

SP Single Track ABS Protection of Trains

On a single track line trains are generally dispatched between locations where sidings can be used to let the trains meet or overtake. The ABS system has three main protection tasks to perform: Protection of opposing trains between sidings, protection of opposing trains when approaching sidings for a meet and protection of trains following each other.

Protection of Opposing Trains Between Sidings

The prime task of dispatching trains on a single track line is to set up meets. There is, however, still the possibility of a dispatching error or the train crew incorrectly passing the place for a meet. To provide some safety for opposing trains means that if two trains for some reason are entering the same single track in opposing direction, the ABS will attempt to bring the trains to a safe stop, or at least warn the trains in advance so that if they collide the collision will be at low speed.

The safest way to handle the protection of trains would be to keep all signals at "Stop" as the normal state, clearing them only when a train needs it and the track is free of conflicting trains. This is what an interlocking or a CTC system does on the command of the tower operator/dispatcher. An ABS system, on the other hand, is fully automatic and has no way of telling where a train is going and must instead create safety for the trains as it detects their movement.

As shown above and below the SP single track ABS has intermediate signals between sidings. Moving from the west siding to the east, the first signal encountered is placed right after the end of the siding, in this document called the Siding-Exit signal. The siding-exit signal protects the line to the next siding, i.e. it only clears when the line is free to the beginning of the next siding. As this signal protects the whole line down to the next siding, 8-10 miles away, it gives no advance signaling (can only show "Clear" or "Stop and Proceed").

About 2-3 miles down the line is another signal, called an Overrun signal. The sole purpose of the overrun signal is to stop a train in case that another, opposing, train is on the line. The overrun signal also only clears when the line is free to the next siding. As the overrun signals are still far from the next siding, no advance signaling is given here either. Note that an overrun signal unexpectedly encountered at "Stop and Proceed" is a serious matter - it may mean that an opposing train is also on the line. The opposing overrun signals must be placed twice a train's stopping distance from each other.

About 1.5 miles from the siding is the Distant signal, advance signaling the aspect on the block signal protecting the siding. Distant signals were originally able to show only two aspects, "Clear" or "Approach", as they only served to inform about the signal protecting the siding. They were then placed 1 mile from the siding and had only the yellow arm. Changed rules later made it illegal for a signal in ABS territory to show a proceed aspect if the line behind it was occupied and so the distant signals had to be rebuilt. The distant signals were equipped with an upper arm and thus became capable of showing "Stop and Proceed". The signals were also respaced to the present distances to allow for the increased stopping distances of longer and faster trains.

When a train enters the line between two sidings i.e. passes the siding-exit signal, the ABS immediately sets all opposing signals down to the next siding to "Stop and Proceed":

This gives protection even for the worst possible situation: Two trains simultaneously moving on to the same section of line:

The trains will both face overrun signals displaying "Stop (and Proceed)" and have plenty of stopping distance. When they finally meet they will be going at restricted speed and thus the situation is not dangerous, though of course quite impractical.

Protection of Opposing Trains when Approaching Sidings

When trains are approaching sidings for a meet they must know which train is to go in the siding and which train is to stay on the main track. Determining the usage of tracks is part of setting up a meet and this information is communicated to the trains when they are informed about the meet.

When trains are approaching the meet from each side of a siding, the siding-exit signals at the ends of the siding will be displaying "Stop and Proceed". If the ABS was to perform like a simple block occupancy system around sidings this would introduce hazards to the trains as the signals protecting the siding (Siding-Entrance signals) will be displaying "Approach". If the trains were arriving at the same time they would both be allowed to enter the block at speed:

To avoid this situation the siding-entrance signals will not only look at the block they're protecting but also at the next track circuit, i.e. to the distant signal on the other side of the siding. The extra stretch of line being supervised is called an overlap, as it extends into the next block:

This way at least only one of the trains can be signaled into the block hosting the siding at speed, the other train will see a "Stop and Proceed" aspect on its siding-entrance signal.

Displaying "Stop and Proceed" for the second train is of little use if the train has passed a signal showing "Clear" shortly before the next signal drops from "Approach" to "Stop and Proceed":

Therefore the circuitry is designed to always provide two signals displaying "Approach" where a train might be approaching from the other side of the siding. This is often called double yellow, referring to the yellow light of a color light signal displaying "Approach".

A train may still see the siding-entrance signal change from "Approach" to "Stop and Proceed" but with double yellow the train will be approaching the signal as if it already was showing "Stop and Proceed".

The siding-entrance signals also supervise the siding switches. The signals drop to "Stop and Proceed" whenever a siding switch is opened (i.e. not lined for the main track):

Protection of Following Trains

The SP single track ABS system only allows for one train at a time on the line between two sidings, though a second train can follow another train at restricted speed. On the lines where this type of ABS is installed traffic density is generally low and thus the need for trains following each other closely is also low.

An SP Single Track ABS Example

To give an example of the above-described features of the SP single track ABS system, we'll look at how a meet could take place on such a line. First an eastbound train is arriving at the meeting point where it is to go in the siding. Note the westbound siding-entrance signal is at "Stop and Proceed" due to the eastbound train occupying the overlap.

The crew stops the train before the siding entry switch and lines the switch for the siding.

The train clears the line

. . . and closes the switch. The meeting westbound train passes its previous siding.

As the westbound train enters the line, the opposing (eastbound) signals change to "Stop and Proceed".

. . . and reclear as the westbound passes them.

Westbound signals stay at "Stop and Proceed" as long as the train is on the line.

The westbound is now approaching the siding with the meeting train. The opposing signals on the west side of the siding are are still providing "double yellow". . .

. . . until the westbound reaches the overlap.

As soon as the westbound clears the track between sidings, the westbound signals held at "Stop and Proceed" change to "Clear", unless another westbound train is following (as it is allowed to do at restricted speed). The eastbound siding-exit signal also clears, permitting the eastbound to leave the siding.

In our example the eastbound stays in the siding for a while. The westbound has reached the other end of the siding and set opposing signals to "Stop and Proceed" on the next stretch of line. The overlap is still active.

After the train clears the overlap, the westbound siding-entrance signal changes to "Approach". The previous signal also stays at "Approach" to provide the "double yellow".

The eastbound now gets ready to depart and opens the switch. This sets the siding-entrance signals at both ends of the siding to "Stop and Proceed" and the eastbound is ready to go.

When opening the switch for the train to depart special rules have to be observed, as a train might be on its way towards the siding and too close to be able to stop when the switch is opened. The crew must wait 5 minutes at the opened switch to see if any train is approaching, before they can pull their train out of the siding.

Normally the eastbound would open the switch as soon as the westbound has cleared the switch. With the westbound moving away in the same block no train can approach from behind, and the clear eastbound signal indicates that no other westbounds are near the siding. The train may therefore pull out of the siding immediately.

Increasing Train Size and Speed

As train lengths and speeds increased over the years so did the necessary stopping distances for the trains. In our example we assumed that trains would be able to stop in the distance from an Approach signal to the next signal. With longer stopping distances signal spacing became inadequate at some locations. Moving the signals farther apart was costly and it might only be a matter of time before the problem would arise again. The solution chosen was to extend the advance signaling to include another aspect, by adding a lamp to the existing signals. The lamp is a special type that flashes yellow when lighted. The yellow flashing light added to a signal displaying "Clear" modifies the signal aspect to "Approach Medium":
Signal Aspect 
SP Rule Name  Indication 
9.1.5 Approach Medium Proceed prepared to pass next signal not exceeding 40 MPH and be prepared to advance on diverging route at the next signal at prescribed speed through turnout.

A train approaching a signal displaying "Stop (and Proceed)" will now first see a signal displaying "Approach Medium", and slow down to 40 MPH before seeing the signal displaying "Approach".

In our example "Approach Medium" could be necessary because the distance between the distant signals and the siding-entrance signals is too short. "Approach Medium" is displayed from the signal before the "distant" signal, i.e. at the overrun signal, when the "distant" signal displays "Approach". The warning distance before encountering a signal displaying "Stop (and Proceed)" has thus been expanded several times.

A similar short distance between signals can be found between the siding-entrance and siding-exit signals. But due to the double yellow feature the distant signal will display "Approach" if not both of the said signals display "Clear", and thus the situation is covered above. Below is shown how "Approach Medium" is used on the overrun signals to give an advance warning of a possible upcoming "Stop (and proceed)" indication:

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Text, Images except for wallpaper, HTML: Carsten S. Lundsten.
Wallpaper photo: Kindly by Eric Schmelz, modified by Carsten S. Lundsten