North American Signaling:

Automatic Block Signaling, ATSF sidings

by Carsten S. Lundsten

Updated 260303 - Document revision history here

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First a few things about lines with ABS:

A single track line with ABS is dispatched using TimeTable and Train Order (TT&TO). In its basic form, TT&TO provides the safety against train collisions, by controlling meets and passes, simply keeping trains separated. Nowadays, Track Warrant Control or Direct Traffic Control are used instead, but serve the same purposes. When ABS is added to the equation, the ABS provides a technical and automatic system to enforce train separation. This raises the overall safety level, and allows the line speed to be raised above 49mph.

In very short terms, the ABS does the following:

The ABS on the 2nd District appears to be the so called "Opposing Overlap" type. This is an older and rather primitive type of ABS, which has in many locations later been converted into an  APB-like type of ABS.

Opposing Overlap ABS, in short, sets up a number of red and yellow signals to protect both ends of the train. More precisely it is 1-2 red signals and 2 yellow signals. By "pushing" this safety zone ahead of itself, two opposing trains will see yellow and red signals early enough to be brought to a safe stop even if traveling at fuull speed. Similarly the rear of the train will be protected by yellow and red signals. Strictly it is only necessary to protect the rear of the train with one red and one yellow signal, but Opposing Overlap ABS cannot determine the direction of the train. This is where APB is an improvement, allowing trains to follow each other closer.

Now for the signaling around sidings. The siding arrangement looks like this:

The switches are spring switches, their normal position being suprvised in the main track signals. The siding is not equipped with track circuits, thus the ABS does not "know" if there is a train in the siding or not. Main track signal are all permissive, i.e. their most restrictive indication is "Stop and Proceed". A train with the proper authorization may proceed past such a signal at red, at Restricted Speed. The signals in the siding are absolute signals, i.e. red means "Stop". These signals may not be passed when red, except after permission from the dispatcher or after other precautions. All signals are green as their normal aspect, i.e. when no train is on main track and switches are normal (the spring switches allow trains to move from siding as well as main track).

And now to the functions of the siding signals when a train passes the siding. As mentioned above, the first sign of an oncoming train (from the left) is signals dropping from green to yellow:

Next will be the signals at the end of the siding dropping to yellow:

As the two signals protect the same track towards the left, their aspects will be the same.

As the train moves closer, so moves the red and yellow signals:

This requires a bit of explanation. The two signals to the left end of the siding are still just displaying the same aspect, but the siding signal to the right dropped to red ("Stop"). This is done to ensure protection of train in th same direction. If there is a train in the siding (which the ABS does not know), this train is now prevented from exiting. Even of the train leaves right at this moment, thus not seeing the signal drop, the oncoming tran is still far enough away to be stopped safely by signals dropping to red/yellow when the main track is occupied and the switch is opened.

The train continues, "pushing" red and yellow signals in front. Opposing signals clear as the rear of the train pass them:

And finally the train clears the "entry" switch:
Note that the signal in the siding, and only the signal in the siding, clears after the train. This is again a matter of protecting trains in  the same direction. Now, this requires a little more explanation: As the ABS cannot see if there is a train in the siding, but it can see a train on the main, there could be two trains present. So the ABS has to decide what to do about the situation. The answer is to try and figure out which direction the train is moving, in this case away from the signals. The ABS therefore clears the siding signal, in case this was a meet. But since it cannot be absolutely sure that the train is going towards the right (for various technical reasons), and the risk that the train stops and changes direction, the ABS must hold the signal on the main red.

Proceeding towards the right, signals clear after the train:

Here the train is far anough away that the signals at the right end of the siding changes to yellow. Since there's no train behind them, thus at most a train in the siding, but simply both change to yellow.

Next interesting change is when signals around the leftmost siding switch clear. When the signal at the point of the siding switch goes back to green, the opposite signal on the main is released and changes to the same aspect as the siding signal, in this case green:

And everything back to normal:

A few more words about the control of the signals at the end of the siding. As shown above, they behave according to the direction of the train. Technically they use the same mechanism in that they monitor the opposing signal at the point of the switch to the siding. If this signal is not green, there's a train present somewhere. The ABS detects whether this train from the one end or the other, thereby determining which way it is traveling. By always keeping one of the signals red it is not a safety problem if the determined direction of travel turns out to be wrong or the train changes direction. This may be a bit tricky to understand and maybe needs another illustration or two...

And now to the siding switches, and their influence on the signals:

In general, ABS signals supervise the switches. An ABS signal governing movement over a switch in an unsafe position, must show its most restrictive aspect. In practical terms, a check for a switch' normal position is included with a check it the track circuit, so opening a switch is that same as occupying the track that the switch is in:

Reversing the switch is in principle the same as opening it. It is, however, possible to gain a little operational advantage by checking for the switch being reversed. A reversed switch is something that happens if a train is lining itself into the siding:
By utilizing the knowledge that the train is heading for the siding, the ABS can disregard the occupied overlap for the "entering" signal in the other end of the siding, and allow this train to clear. Thereby letting the meeting train come at a higher speed than "Stop and Proceed" would allow.

Without trains to blur the effect of the open switch:

Can you figure out why the rightmost siding signal drops to red?

Comments, corrections and more information about ABS systems are very welcome. Email me at

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Text, Images, HTML: Carsten S. Lundsten.