North American Signaling:

Automatic Block Signaling, ATSF "Flat Pair" ABS

Created August 18 2007 by Carsten S. Lundsten

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A number of railroads, including the Atchison, Topeka & Santa Fe Railway, implemented a special variety of single track Automatic Block Signaling, that the ATSF refered to as "Flat Pair" signaling. "Flat Pair" refers to the intermediate block signals standing back-to-back in pairs, as opposed to the "Staggered Pairs" on the older Opposing Overlap system (I'll get around to describing that eventually...). The "Flat Pair" ABS is technically in essence an Absolute Permissive Block (APB) signaling , but consistent with practices on (some) western roads it had all permissive signals. In later years, however, the number plates have been removed from the headblock signals, thus making them absolute signals as on an ordinary APB.

What technically differentiates the "Flat Pair" ABS from ordinary APB is the arrangement of signals around sidings. "Flat Pair" ABS features an arrangement of signals around sidings, with automatic functions for meets and passes, that are unique from ordinary APB. The arrangement is also found on the ATSF version of the older Opposing Overlap ABS (which I will some day get around to explain in another document...). I have seen similar systems in operation on lines formerly by the Texas & Pacific and the Southern Pacific.

Before reading further, it is recommended to become familiar with the method of operating trains on single track, the "tumble-down" and the protection of opposing trains around sidings on the standard Absolute Permissive Block (APB) signaling .

The arrangement of signals around a siding in ATSF Flat Pair territory is as shown below:

The siding switches are spring switches, their normal position being supervised 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. As mentioned above, the signals on the main track 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, however,  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).

In order to illustrate the features of ATSF Flat Pair ABS, let's follow a train across a section of a Flat Pair line:

To the left is one end of a siding, then a section of intermediate block signals, then another siding and a piece of the line towards the next siding to the right. As with most ABS most signals are green when the block is at rest (i.e. no trains nearby).

First, a train is approaching from the left. For now it has not progressed past the siding switch at the left siding, so no tumble-down has taken place yet:

As the "Headblock Signal" is located before the siding switch, occupying the switch occupies the first block, and  thus triggers the tumble-down:

Please note what happened at the next siding: Not only did the Headblock signal drop to red, but so did both signals in the siding (the Leave Siding signals). The leftmost Leave Siding signal is of course a mirror of the Headblock signal on the main track (as they protect against the same train movement from the left), but the rightmost Leave Siding signal deserves an extra bit of expanation.

In the situation above, the rightmost Leave Siding signal establishes protection against a train that has gone in the siding to be overtaken. Remember that in Automatic Block Signaling territory, trains in the same direction are to be protected by the signals alone, and this is the reason why the signals in the siding needs to be absolute signals. By dropping the signal to red, our train from the left is protected against any train leaving the siding. And by establishing the protection so early there is enough time that a train in the siding can accept the Leave Siding signal just as it drops, without endangering our train train coming from the left on the main track. This principle is illustrated in my document on ATSF double track ABS .

As the train moves across the line, the signals clear behind it (as on ordinary APB). Note that the Headblock signal and the Leave Siding signal at the leftmost siding share the same aspect:

There are several different ways to build the circuits that detect the direction of the train. These circuits decide when to tumble signals down and when to enable the signal in the rear to clear up after the train. One of these varieties of circuits keep the opposing intermediate block signal behind the train at red, until the train has cleared the block. This seems to be the prefered implementation that the ATSF used. The choice of circuits seems to be a matter of preferences at the railroad or the manufacturer, but keeping the rear opposing signal at red does prevent a reversing train from picking up much speed before it has triggered the tumble-down in the opposite direction.

The train continues, now approaching the overlap section for the Entering signal by the siding:

Then entering the overlap:

And finally the train clears the "entry" switch...

...and then clears the Leave Siding signal behind it to let out any meeting train in the siding. The signal on the main track stays at red, thus protecting a train leaving the siding:

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. At the same time, 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:

And everything back to normal:

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 (i.e. not lined to the Main Track), 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 section 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 approach at a higher speed than "Stop and Proceed" would allow.

Without trains to blur the effect of the open switch:

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Text, Images, HTML: Carsten S. Lundsten.