In the late 1920s the Pennsylvania Rail Road (PRR) installed Absolute Permissive Block (APB) signaling on several lines in Ohio. This document describes the PRR APB inplemented on those lines, based on an article in the June 1929 issue of "Railway Signaling".
The reader is assumed to be famiiar with the principles of basic Absolute Permissive Block signaling. The PRR APB is a fairly straight forward version of basic APB but is adapted to PRR Position Light signals and in addition features a slightly different overlap handling scheme at sidings
In its default state, i.e. with no trains nearby, an APB line typically looks like this:
The PRR Position Light signal aspects and indications relevant for APB are:
| Signal Aspect | PRR Rule | Name | Indication |
|---|---|---|---|
  |
281 | Clear | Proceed |
  |
285 | Approach | Proceed prepared to stop at next signal, trains exceeding Medium speed must immediately reduce to that speed |
 |
291 | Stop and Proceed | Stop, then proceed at Restricted speed |
 |
292 | Stop | Stop |
Headblock signals were a single head ("single arm" in PRR terms) and lacking a number plate. Permissive signals had a marker light below, allowing their most restrictive aspect to become "Stop and proceed", and had number plates. The signals were approach lit but for clarity shown as permanently lit in the illustrations in this document. The signals were lit by the occupation of their immediate approach track circuit.
Each block was (as on standard APB) divided into two track circuits to allow the APB system to detect the direction of a train entering a block. A standard block was approximately 2 miles long. On the PRR APB the blocks bordering the sidings were shorter (app. 1 mile) in order to reduce the runtime effects of the "double yellows" and to allow trains being overtaken to exit the siding quicker after the passing train. It seems, however, that these shorter blocks have not been a key design feature and were not found on all lines.
Besides the above mentioned optimizations of signal spacing, which doesn't affect the system functionality as such, the PRR APB features a non-standard function to optimize signal overlaps at sidings. The function is designed to allow smoother meets. Imagine a situation where two trains are approaching a siding at approximately the same time. The train traveling towards the left is the first to arrive and is the one to hold the main track. Initially the block along the siding is open for both trains but as in standard APB there is a double "Approach" indication in both directions.
As the trains close in they both pick up an "Approach" on the last intermediate block signal before the siding and the signals protecting the station block are still at "Approach" for both trains. The illustrations below show the extend of the signal overlaps, i.e. how far ahead a signal "looks". The situation is illustrated both for standard APB as well as for the PRR APB. So far both systems respond identically.
 |
 |
The first train to hit a signal overlap is the leftbound train. When the overlap becomes occupied the rightbound block signal protecting the station block drops to a "Stop then Proceed" indication. Again both systems respond identically as far as signal indications go but the PRR APB detects that the leftbound train came first and shortens the overlap for its signal.
 |
 |
When the 2nd train, the rightbound, passes the middle of its block a standard APB drops the leftbound station block signal in the face of the leftbound train. The train is running on an "Approach" indication but may at this point have seen the signal already. Either way, if the leftbound train as in this example is to hold the main track it will have to make an unnecessary stop due to the signal dropping in its face and then proceed at Restricted Speed through the station block. The PRR APB system on the other hand holds up the "Approach" indication as long as the station block itself is clear and thus allows the leftbound train a smoother arrival.
 |
 |
This optimized overlap function is only relevant in the this particular situation. If the first train makes it past the station block signal before the second train reaches the midsection point on the other side, or if the first train is the one to take siding, the two systems will yield the same result.
To give an example of the above described features of an APB system, we'll look at how a meet could take place on the Pennsy APB line. First a leftbound train entering the scene at the preceding siding. The leftbound train is set to use the main track at the meet.
Then it enters the line and triggers the tumble-down and leaves the previous station.
A little while later a rightbound train tumbles its line towards the meeting point.
The two trains close in on the meet. They have not yet reached the overlaps.
Now both trains are occupying the default overlaps but due to the mechanism described in detail above the first train, the leftbound, shortens its overlap and thus the train retains its "Approach" even when the rightbound train later passes the center of its block.
The shortened overlap allows the leftbound train to arrive at the left end of the siding at speed, allowing it to be ready to depart whenever the rightbound is in the siding or maybe even help it open the switch for a quicker entry.
With the rightbound train in the siding the leftbound can depart once the switch is lined back to normal again. For the sake of the example the rightbound train stays in the siding for now.
The overlap reduction mechanism is only in effect when both trains are approaching the station. Not in a situation like this even though it was active on arrival and it could have come in handy if a 2nd leftbound had been close behind. Instead the ordinary overlap must be cleared.
Finally, just to illustrate the effect on nearby signals, this is what it looks like when our rightbound train opens the siding switch to depart. As with standard APB special rules, including waiting time, has to be observed when opening a switch as another train might be on its way towards the siding and too close to be able to stop when the switch is opened.
