North American Signaling:

Absolute Permissive Block

Updated November 7 2017. By Carsten S. Lundsten


An Absolute Permissive Block Line
APB protection of trains
     Protection of Opposing Trains between Sidings
     Protection of Opposing Trains when Approaching Sidings
     Protection of following trains
A complete APB system example

This document explains the functions of the Absolute Permissive Block (APB) systems in use on single track lines across much of North America. The APB is a special type of Automatic Block Signaling (ABS). Different railroads and different manufacturers have implemented different features in their APB systems but this document explains a common version that is often seen in textbooks. APB was conceived by the General Railway Signal Co. of Rochester NY around 1910 and quickly became the most common type of single track ABS to be installed. APB is probably the most common type of Automatic Block Signaling (ABS) for single track lines in North America.

Contrary to many of the single track ABS systems in use at the time APB not only protected trains running in the same direction but it also provided protection between opposing trains. On any single track ABS, including APB, protection between movements in opposing direction has to be provided by the dispatch system. Back then dispatching was done via TimeTable and Train Orders (TT&TO) later by Track Warrant Control (TWC), Direct Traffic Control (DTC), Occupancy Control System (OCS) or similar. Regardless of the dispatch system in question any train has to have an Authority to occupy a main track. The Authority comes from the dispatch system but the ABS system provides an extra level of safety in case the dispatching fails (either by mistake of the dispatcher or a train crew) and two trains end up heading against each other on the same track. Where the original single track ABS systems only provided a partial protection between opposing moves the APB provided a full such protection.

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 remote control of an APB line. Switches are hand thrown by the train crews as needed though some variations of APB feature spring switches at the ends of sidings. 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 sidings are not track circuited and the APB only responds to conditions on the main track.

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, however, irrelevant/unimportant signals are shown shaded.

In this document BNSF signaling aspects and indications are used. The signal aspects and the rule numbers vary by the individual railroads but the signal indications are quite universal for APB installations across North America. The BNSF signal aspects used here are very typical for many railroads:
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

The protection of trains that the APB-system provides can roughly be divided into 3 categories:
- Protection of opposing trains between sidings
- Protection of opposing trains when approaching sidings for a meet
- Protection of trains following each other.

Protection of opposing trains between sidings

On a single track line trains are generally dispatched between locations where trains meet can or overtake, i.e. locations with sidings. The APB system is therefore primarily designed to handle train movements from siding location to siding location. Trains that turn back between sidings usually require special measures to be taken to ensure safety. At locations where this happens regularly (for example by trains serving local industries) the signal system will usually to some degree be adapted to support such operations.

The dispatch system determines where trains will meet or overtake and is responsible for the primary safety between opposing trains. Which means that opposing trains may never be authorized on the same main track (at least not if doing more than Restricted Speed). An Automatic Block Signal system does not authorize train movements. A train without an authority to proceed must stop even if it sits right at a signal at "Clear". An ABS or APB system is fully automatic and has no way of telling whether/where a train is going; it must instead establish safety entirely from detecting the location and direction of trains.

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. The signals must be at least the stopping distance of a train apart. Typical signal spacing is 1-2 miles.

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". This mix of absolute and permissive signals gave name to the Absolute Permissive Block signal system. In their normal state all signals show "Clear".

The main characteristic of the APB is that 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 at green for the same section of line. The trains will both face intermediate signals displaying "Stop (then Proceed)" and have plenty of stopping distance. When the trans finally meet they will be going at restricted speed and thus the situation is not dangerous. But of course quite impractical. A minimum of 4 blocks between sidings are necessary to ensure safety:

If it is possible to establish the middle block long enough to accomodate the combined stopping distances for both trains (for example if line speed is low), only 3 blocks are necessary between the sidings. The outer 2 blocks may be shorter but still need to be at least a stopping distance long:

If the distance between 2 sidings only allows for 2 blocks then a technically simple solution is to repeat the ""Approach" in the intermediates signals between the sidings into the headblock signals. The result is an "Approach" in the headblock signals when trains enter the line thus the trains are prepared for the intermediate signals in the middle to be at "Stop and Proceed". While this is an easy modification of the circuits it reduces traffic capacity for trains following each other in the same direction, more about that later.

Another solution to the problem is to tumble the signals earlier by introducing an overlap section ahead of the headblock signal. In the early tumble-down scheme the tumble-down is triggered when a train enters the overlap. The overlap is marked by a sign behind which a train on the main track will have to stay if an opposing train is approaching, otherwise it will tumble the signals in the face of the oncoming train:

The length of the overlap must be long enough to allow time for the tumble-down and sufficient time for an oncoming train to observe the headblock signal drop to red.

Protection of opposing trains when approaching sidings

When trains are approaching sidings for a meet they will know from the dispatching system which train is to go in the siding and which train is to stay on the main track. The APB, however, will not "know" where the trains are going and it is required to protect the trains either way.

When trains are approaching the meet from each side 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 Entering Signals protecting the siding will both be displaying "Approach". If the trains were arriving at the same time they would both be allowed to enter the same block at speed:

To avoid this situation the entering signals 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:

In this manner only one of the trains can be signaled into the block 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, however, of little use if that train has previously passed a signal showing "Clear" shortly before the entering eignal 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 entering signal change from "Approach" to "Stop and Proceed" but with double yellow the train will be approaching the signal prepared to stop.

The entering 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

Movement Authorities in ABS and APB territories may overlap for trains running the same direction, i.e. several trains may have Authority to the same piece of track. It is then the task of the ABS/APB to separate the trains safely, i.e. to protect the rear of the train with a block signal showing "Stop (and Proceed)" or "Stop" while still providing the protection against opposing trains. The tumble-down feature in itself of course serves both purposes but it is more than necessary to protect the rear of a train. For the rear of a train it is enough to have only one "Stop (and Proceed)" signal (and the previous signal at "Approach"):

The APB system therefore contains a direction sensing circuit that enables it to detect the direction of a train in a block. If a block signal protecting an occupied block has detected that the train is moving away it will not tumble the previous signal but rather allow it to show "Approach" (and signals further away "Clear"):

Trains can follow each other with block signal spacing but in order to maintain running speed there needs to be somewhat more than 2 blocks of separation between them in order for the 2nd train to avoid the restriction of an "Approach" signal:

Double yellow on approach to sidings was in early APB installations switched off for following trains but later this directional function seems to have gone out of fashion for standard APB. The double yellow feature on approach to sidings extends this theoretical minimum separation of trains to 3 blocks:

The "triple yellow" solution to short distances between sidings separates trains even further:

Train speeds over the line varies due to speed restrictions, grades etc. and in reality trains need to be separated even further than the examples above.

A complete APB system example

First an eastbound train is aproaching a meeting point where it is to go in the siding. The train has entered the last block before the siding but not yet occupied the overlap for signal 41. Signals 41 and 31 are displaying the "double yellow":

When the overlap gets occupied Entering signal 41 drops to "Stop and Proceed":

The crew stops the train before the siding entry switch and lines the switch for the siding. Which drops Entering signal 52 to "Stop and Proceed". In the meantime the westbound train is approaching. So far it is east of the next siding and signals 12 and 2 are now displaying the "double yellow".

The eastbound train clears the line...

...and lines the switch back. The eastbound train stays in the siding because it is instructed to meet another train at that location. The westbound train passes the siding east of the meet:

The westbound passes Headblock signal 1 and the opposing signals until Headblock signal 42 tumble down to "Stop (and Proceed)":

The opposing signals clear as the westbound passes them:

Note that signal 1 changes to "Approach" to allow any following train to proceed as closely behind as possible:

The westbound is now approaching the siding with the eastbound train. The opposing signals 62 and 52 on the west side of the siding are are still providing "double yellow":

...until the westbound reaches the overlap and Entering signal 52 drops to "Stop and Proceed":

As soon as the westbound clears the line, the eastbound Headblock signal 42 clears. If the eastbound has Authority to proceed is can now open the switch and pull out. For the sake of this example the eastbound stays in the siding for a while longer:

The westbound reaches the other end of the siding and tumbles opposing signals signals on the next stretch of line:

And clears the overlap:

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

The eastbound train now gets ready to open the switch and depart. Before opening a main track switch in ABS territory the train crews have to make certain that no train is approaching. Obviously their Authority should guard against opposing trains but there is still the possibility of an error. And there might be another eastbound with overlapping Authority to the same track.

Headblock signal 42 is "Clear" so obviously no opposing train is on the way. If the eastbound train crew can see Headblock signal 51 being "Clear" they would be sure no following train is coming either. Alternatively they might be able to see Entering signal 41 being "Clear" which would also indicate that nothing is coming from behind. If the signals are approach lit it may not be possible to deduct anything from the signals. Had the eastbound been ready when the westbound train had just passed the crew would obviously know that nothing was coming from behind and Headblock signal 42 would be lit and "Clear".

If the eastbound train crew cannot determine for sure if a train is approaching a crew member must open the switch. This drops signals 41 and 52 to "Stop and Proceed" but may be too late if a train is too close. The eastbound train must then wait a predetermined amount of time (5-10 minutes) with the crew member ready to restore the switch if something shows up:

When the time is up and eastbound finally pulls out onto the line it tumbles the opposing signals:

After lining the switch back to normal the eastbound is ready to go.

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Text, HTML: Carsten S. Lundsten.