Railroad Signalling: Automatic Permissive Block

Railroad Rules, Signalling, Operations:

Absolute Permissive Block

Carsten S. Lundsten

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This document explains the Absolute Permissive Block (APB) systems in use on single track lines across much of the USA. The APB described in this document is a typical installation, some variations may be found on various railroads. APB is a special type of Automatic Block Signaling (ABS) for single track. It is special in that it provides full signal protection for opposing trains, in addition to the protection for following trains that ABS always provides. Other single track ABS systems only provide full signal protection for trains following each other while opposing trains must be protected by some other system, often a track occupation authority system (Timetable/Train Orders, Track Warrant Control, Direct Traffic Control, Occupancy Control System etc.). While most single track ABS systems do provide some protection against opposing trains, the protection is not complete, thus the need for track possession authority as well.

Though the APB provides full safety by itself, trains still need to be dispatched. The safety of the APB system allows for an informal way of dispatching but most railroads prefer to use a system like Track Warrant Control for dispatching, probably because the railroads already uses this system extensively on unsignaled lines or lines with plain ABS.

An Absolute Permissive Block line:

The Absolute Permissive Block 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 APB line. Switches are thrown by the train crew as needed. APB lines may span hundreds of miles without any controlled signals.

An APB line typically looks like this, shown in its normal state without any trains present. Note that all signals show "Clear":

Each block is divided into two track circuits to allow the APB system to detect the direction of a train entering a block, by sensing which track circuit becomes occupied first. The use of the direction sensing is described below. The APB only responds to conditions on the main track and the sidings are thus not track circuited.

Signals on lines like this may be approach-lit. This means that when no train is approaching a signal the lamps are switched off to increase their lifetime. A signal is switched on only when a train enters the block leading to the signal. In this document, however, all relevant signals are shown as if they were constantly lit to illustrate the block functions. In some examples, irrelevant/unimportant signals are shown shaded.

In the examples in this document BNSF signaling aspects and indications are used as APB is/was used on some former BN lines.. Except for the rule numbers these signal aspects and indications are universal across the United States:

Signal Aspect BNSF Rule Name Indication
9.50 Clear Proceed
9.54 Approach Proceed prepared to stop at next signal, trains exceeding 40 MPH immediately reduce to that speed
9.56 Stop and Proceed Stop, then proceed at restricted speed
9.61 Stop Stop

APB 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 APB 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. To provide full safety for opposing trains means that even if two trains for some reason enter the same single track in opposing direction, this situation must be handled by the APB system in a safe way.

The safest way to handle the protection of trains would be to keep all signals at "Stop" as the normal state, and to clear 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 or APB system, on the other hand, is fully automatic and has no way of telling where a train is going; it must instead create safety for the trains as it detects their movement.

As shown above the APB has intermediate block signals between sidings. These signals serve both to separate trains following each other and as a means of stopping opposing trains.

At the ends of the sidings are the signals termed headblock signals. These signals indicate whether it is safe to proceed to the next siding. The headblock signals are absolute signals; i.e., their most restrictive aspect is "Stop" (and stay). Their absolute nature is indicated in some way, typically either by the absence of a number plate, or by the addition of an "A" ("absolute") plate. The signals between sidings are permissive signals whose most restrictive aspect is "Stop and Proceed". In their normal state all intermediate signals (i.e. signals between sidings), and headblock signals show "Clear" as shown above.

When a train passes the headblock signal and enters the track between two sidings, the APB sets all opposing signals down to the next (opposing) headblock signal to red, the so called tumble down:

This gives protection even for the worst possible situation: Two trains simultaneously passing the headblock signals for the same section of line:

The trains will both face signals displaying "Stop (then 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. If block lengths are less than twice the stopping distance of a train, a minimum of 4 blocks between sidings are necessary to ensure safety (as shown). If block lengths are at least twice the stopping distance long, only 3 blocks are necessary.

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. As the APB is required to ensure safety for the opposing trains also when approaching a siding for a meet, some special measures must be taken.

When trains are approaching the meet from each end of a siding, the headblock signals at the ends of the siding will be displaying "Stop". If the APB was to perform like a simple block occupancy system around sidings this would introduce hazards to the trains as the signals protecting the siding 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 signals protecting the siding will not only supervise the block they're protecting but also the next track circuit, i.e. first half of the next block. 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 signal.

Displaying "Stop and Proceed" for the second train (the train on the left of the illustration) 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" before a headblock signal displaying "Stop". This is often referred to as double yellow.

A train may still see the signal protecting the siding 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 signals protecting the siding 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

Trains following each other on the single track only needs to be separated by a block signal showing "Stop (and Proceed)". As mentioned earlier the APB system contains a direction sensing circuitry that enables it to detect the direction of a train in a block. This direction detection is used to determine whether an occupied track circuit should tumble down the signals leading to the block (i.e. which signals are opposing) or merely protect the block by a single signal showing "Stop (and Proceed)".

An APB example

To give an example of the above described features of an APB system, we'll look at how a meet could take place on an APB line. First an eastbound train is arriving at the meeting point where it is to go in the siding. Note that the westbound signal protecting the siding 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 eastbound train stays in the siding because it is instructed to meet another train at that location. The meeting westbound train passes its previous siding.

The westbound passes the headblock signal and the opposing signals tumble down.

The opposing signals clear as the westbound passes them.

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 line, the eastbound headblock signal 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 tumbled down the signals on the next stretch of line.

The overlap is also active for following trains in this example. In some installations it is switched out by direction sensing circuitry.

After clearing the overlap, the westbound signal protecting the siding changes to "Approach". The previous signal also stays at "Approach" to provide the "double yellow".

Finally the westbound clears the first block after the siding and the "double yellow" signals change to "Clear".

The eastbound now gets ready to depart and opens the switch. This sets the signals protecting the siding to "Stop and Proceed" and the eastbound is ready to go.

When opening the switch for the train to depart special rules has 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 be approaching from behind, and the clear eastbound headblock signal indicates that no other westbounds are near the siding. The train may therefore pull out of the siding immediately.

Comments, corrections and more information about APB systems are very welcome. Email me at csl@sandes.dk

Text, Images, HTML: Carsten S. Lundsten

Last updated: 1997-11-08