North American Signaling:

ATSF double track Automatic Block Signaling

by Carsten S. Lundsten. Updated September 26, 2000

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Signal Aspects and Indications

Following Trains on basic ABS
Overtaking Trains
Example of ABS behavior

This page explains the Automatic Block Signal (ABS) systems in use on former ATSF double track lines. By double track is meant that Main Tracks are normally used only in one direction. Trains operating in this direction are said to run "with the current of traffic". A Double Track ABS system handles the safety of trains following each other with the current of traffic. Operations against the current of traffic requires a safe dispatching method to be established on the affected track. On the lines describer herein, Track Warrant Control (TWC) is superimposed on the ABS. During normal operations, however, the TWC is not necessary and is oputside the subject of this document.

The ABS is a fully automatic. There is no remote (human) control of signals or switches and there is no direct supervision of the line. The dispatcher keeps track of the location of trains by instructing the trains to report their positions as necessary.

Signal Aspects and Indications

On the former ATSF, ABS/DT block signals can display 4 signal aspects:
Signal Aspect  Rule  Name  Indication 
9.50  Clear  Proceed 
9.54  Approach Medium  Proceed; approach next signal not exceeding 40 MPH and be prepared to enter diverging route at prescribed speed 
9.56  Approach  Proceed prepared to stop at next signal, trains exceeding 40 MPH immediately reduce to that speed 
9.61  Stop then Proceed  Stop, then proceed at restricted speed 

Following Trains on basic ABS

On each track, the ABS has Automatic Block Signals for trains running with the current of traffic. On most double track lines, this means signals for right hand running only. When no trains are present, all signals display Clear. This is the normal state of the block system:

Actually, most signals on double track like this are approach-lit. This means that when no train is approaching the signal the lamps are switched off to increase their lifetime. A signal is switched on only when a train enters the block leading up to the signal. In this document, however, all relevant signals are shown as if they were constantly lit to illustrate the block functions.

As in any ABS system, a train is protected from the rear by a signal displaying "Stop and Proceed". This means, that a following train can only approach from the rear at Restricted Speed, i.e. slow enough to stop for any obstruction. In order to ensure sufficient braking distance for a following train to the signal at "Stop and Proceed" , the two previous signals will give advance warning, the first by displaying "Approach Medium", flashing yellow on ATSF, the next "Approach", steady yellow:

Overtaking Trains

The Needles Subdivision (Barstow - Needles) on the former AT&SF mainline has 145 miles of TWC/ABS/DT between Daggett and Ibis. On such a line, with fairly high traffic density and long grades, it is necessary to have a way of letting trains be overtaken. For this purpose there are numerous sidings along the line. Trains usually get a Track Warrant permitting them to run all the way from Daggett to Ibis, and the dispatcher will instruct trains verbally to enter a siding if they need to be overtaken.

Adding sidings to the line undermines the ABS system's ability to protect following trains unless further action is taken. A train may enter the siding, leaving the main track clear and all signals green, and then pull out of the siding just in front of a train on the main. Here is an illustration of this potential danger:

Rules cover this situation by saying that if a train has to leave the siding and enter the main track, the switch to the main track (the siding "exit" switch) must be opened. This action sets the signal protecting the switch to "stop" and sets up signal protection for pulling out of the siding. The crew must then wait 5 minutes to see if a train shows up (i.e. a train that was too close when the switch was opened and the protecting signal changed to red). To be able to close the switch for the approaching train the crew must wait at the switch. If no other train can be seen or heard after 5 minutes, the train can pull out of the siding.

To avoid having to wait 5 minutes after the overtaking train has passed, the former ATSF lines have been equipped with a special "Leave Siding" signal, placed at the end of the siding. The Leave Siding signal tells an engineer of a train in the siding whether it is safe to pull out on the main or not. As the ABS system is not controlled by a dispatcher, the Leave Siding signal cannot determine whether the train should leave the siding, only if it is safe to do so.

The Leave Siding signal can only display two aspects:
Signal Aspect  Rule  Name  Indication 
9.56  Approach  Proceed prepared to stop at next signal, trains exceeding 40 MPH immediately reduce to that speed
9.62  Stop  Stop 

An important thing to note is that while block signals on the main track display "Stop and Proceed" as their most restrictive indication, the Leave Siding signal shows "Stop". This way a train in the siding is bound to wait for the signal to clear before pulling out of the siding, thus preventing it from pulling out in front of a train approaching from the rear on the main track.

As the next block signal on the main usually is located shortly after the end of the siding, there is no need for the Leave Siding signal to provide advance information for that signal. The Leave Siding signal is normally approach-lit by a track circuit covering the last 400 yds (370 m) of the siding (not shown on drawings). This track circuit has no other purpose. Below is shown an arrangement of sidings for both directions, together with a crossover. The crossover is hand-operated and only used in special situations. Switches designated "S" are spring switches:

The Leave Siding signal protects the block in which the siding ends from a train coming off the siding. If a train on the main is closer than stopping distance from the end of the siding, the Leave Siding signal will display "Stop". In practical terms this means that the Leave Siding signal will display "Stop" if any one of the 3 main block signals  before the end of the siding displays "Stop". This gives protection even in the worst case, which is:

Both trains have acted according to signal indication. As soon as the train from the siding occupies the main, the block signal in the rear  will switch to "Stop and Proceed". With the previous block signal switching to "Approach", the rear train has a full block of stopping distance (2 miles) plus the length of the siding ; typically at least half a block.

Example of ABS behavior

This is how the signals act when a train on the main track runs by a siding:

When a train is to be overtaken, it is instructed by the dispatcher to take the siding at location X. The train stops before the entry switch to the siding and the conductor aligns the switch to the siding. Opening the switch sets the protecting signal to "Stop and Proceed" if the train has not already done so by occupying the block (the entry switch to the siding need not be located immediately behing the protecting signal). The train then pulls into the siding at restricted speed and stops when the last car is in the siding. The conductor closes the siding entry switch (aligns it to the main track), returning the signals to proceed aspects. The conductor then walks back to the engines.

Previously, when cabooses were still used, the switch would be opened by the front-end brakeman, the train would proceed into the siding, and then the switch would be closed by the rear-end brakeman or conductor riding in the caboose. Since the demise of cabooses, the brakeman or conductor must wait until the train gets into the siding, close the switch, and walk to the front end.

When the overtaking train approaches, the signals will work as sketched before. The ABS system does not need to "know" if there is a train in the siding or not.

As soon as the overtaking train has cleared the block where the siding ends, the Leave Siding signal displays "Approach", allowing the train to pull out. The siding exit switch is spring loaded, so the train can just pull out as fast as the switch, track conditions and block signals ahead permit.

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Text, Images, HTML: Carsten S. Lundsten.