Railroad Signalling: PRR ABS

Railroad Rules, Signalling, Operations:

PRR Automatic Block Signaling

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John Cooper,
for comments based on his own observations
Jon Roma, Carsten Lundsten
for helpful commentary


PRR ABS Aspects

A full description is found elsewhere.

PRR Interlocking and Distant Aspects

The interlocking aspects used in this document are shown below. A train would receive an Approach Medium aspect at the signal previous to the one where Medium Clear is received. An Approach signal warns of a Stop signal.

Signal Aspect Rule Aspect Name Indication
283 Medium-clear Proceed; Medium Speed through interlocking limits.
292 Stop-signal Stop.

"Lamp Test"

If one could magically turn on all lamps, the line would look like the following. Note the 'incomplete' set of lights on the easternmost 2 signals of #1 track (the lowest track).

The normal state, with no trains present, is as follows. Note the following:

  1. PRR numbered tracks from south to north in sequence, i.e., #1, #2, #3, and #4 from bottom to top. The 3-track section at right could be numbered #1-#2-#3, or possibly for historical reasons #2-#3-#4 (if #1 track had been recently removed).
  2. Intermediate automatic signals all have number plates. Their aspects are governed only by occupancy of next 2 blocks (or 3 blocks in 4-aspect signalling). There is no human control. The default aspect is Clear unless modified by track conditions or other signals in advance (farther 'forward' along the line). The most restrictive aspect is Stop-and-proceed.
  3. Interlocking home signals have no number plates. They normally remain at Stop until 'brought up' by the tower operator. When 'brought up', they are governed by block occupancy of the next 2 (or 3) blocks but also by the position of switches in the interlocking. These signals are absolute, with the most restrictive aspect being Stop (and stay).
  4. Interlocking Rules (IR) govern the portion of track between opposing home signals of each interlocking. All tracks into the interlocking have a signal (but note that a westbound train on #1 track would encounter no intermediate automatic signals -- more below.
  5. Automatic Block System (ABS) rules govern the portion of track between the 2 interlockings.
  6. The traffic lever on #3 track is set westbound in each tower. This causes the intermediate signals to display Stop and Proceed for eastward moves, and aspects less restrictive than Stop and Proceed for westward moves.
  7. Typically, main line blocks (between automatic signals) are on the order of 2.5-3 miles (4-5 km) in length in keeping with today's longer trains. Previously, 1.5-2 miles (2.5-3 km) or even less was typical. An interlocking of the size depicted here would typically span 0.5 mile (1 km).

Simple sequence - no other trains present

Here is the sequence of signal changes as a train proceeds from the interlocking at the left to the interlocking at the right. As drawn here, towers are still present at both interlockings, and the tower operators control access into the ABS sections. The tower operator has brought up the home signal for #1 track, which shows Clear. Note that this signal now serves as the block signal for the entire distance between it and the automatic signal in advance. There is no signal at the exit of the interlocking.

The home signal drops to Stop soon after the train passes it and remains so:

The train is still seen on the model board of the western tower but is now in ABS territory.

The train is no longer on the model board of either tower. If this section is instead run by a centrally located dispatcher, the dispatcher panel may have an occupancy indication for each intermediate block, or the panel may have only one light for the entire inter-interlocking stretch.

The train is now stopped. It is in ABS territory, outside interlocking limits. It may now be held where it is to allow another train to pass it eastbound or westbound.

Summary of aspects

Thus, a train running under normal circumstances will receive the following aspects:

--> --> or

In 4-aspect (3-block) signalling, it will instead receive:

--> --> --> or

Route established through both interlockings

If the tower operator at the more easterly of the 2 interlockings is alert and has no conflicting moves, he will "bring up" the signal on #1 track. His model board with then show:

. . . and the signal sequence will then be:

The train now receives an Approach Medium signal. This signal requires a speed reduction to Limited Speed (40 MPH, 65 km/h; or 45 MPH (70 km/h) for passenger trains) immediately and warns of Medium Speed being needed at the interlocking coming up. In Cab Signal territory, the cab signal aspect drops to Approach Medium as well. This downgrading of allowable speed requires an acknowledgement [and a minimum brake application?].

The train shows up on the model board of the eastern tower. The trains receives a Medium Clear signal, indicating that though the route is clear of trains, a medium-speed (30 MPH; 50 km/h) route is selected. In Cab Signal territory, the cab signal will continue to show Approach Medium (the fixed signal governs, of course).

As the train passes the home signal, this signal drops to Stop. The cab signal will go back up to Clear.

At which point the model board in the eastern tower will look like this:

The head end of the train passes the opposing home signal:

. . . and it ultimately passes out of interlocking limits entirely:

Train following another train

Trains passing

Running against the current of traffic

Text, Images, HTML: Mark D. Bej
Last updated: 1997-11-03