Railway Signalling and Operations FAQ: Automatic Block System

Railway Signalling and Operations

Automatic Block System


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Acknowledgement


Notes

This document describes Automatic Block System, ABS, in the traditional meaning of that term. Some railroads, particularly those under current versions of the General Code of Operating Rules, use the term ABS for what I feel is better named Absolute-Permissive Block (APB). I gather that those railroads no longer have "traditional" ABS territory, having replaced it with "CTC territory.

Finally, I intend "soon" to replace the ASCII drawings with gifs. Please be patient. Bug me if I forget.


Automatic Block System

Automatic Block System is a signalling schema used to increase the capacity of a given track by allowing movements (trains) to follow one another, reasonably closely, on the same track, while providing safety. The signals used in ABS to accomplish this goal operate automatically (i.e., automatic signals) and allow trains to pass at Restricted Speed (i.e., are permissive signals).

Clearly, this means that ABS does not provide for control over trains by a tower operator or dispatcher. This control must be exercised by other means -- means which will be the topics of other documents.

Some historical information may be in order. Prior to the invention of ABS, railroads could only control trains at manned locations. These manned locations were train stations, interlocking towers, and so forth. These locations were the only places where instructions could be given to the trains, or where any control could be exercised.


      ---------                                                   -<BBBBB---
     /         \                                                 /          \       
-------------------------------------------------------------------------<AAAAA--------
        ABEL                                                        BAKER

The problem is that, once either train A or train B is ready to leave Baker station, the entire track from Baker to Abel is blocked. Let's say that train A is the higher priority train and it leaves first:


      ---------                                                   -<BBBBB---
     /         \                                                 /          \       
---------------------------------------------<AAAAA------------------------------------
        ABEL                                                        BAKER

Train B must now wait for some specified time before leaving Baker under flag protection. Or in later (telegraph) days, train B might wait until train A arrives at Abel and the station agent or tower operator there notifies the agent or operator at Baker that train A arrived. That is, train B must wait for the block between Abel and Baker to clear.

Clearly, much time is being wasted in waiting for this to happen. This could be remedied by breaking up the long block between Abel and Baker into smaller blocks. Each block could be made one braking distance in length:


      ---------                                                   -<BBBBB---
     /         \                                                 /          \       
----|-----------|--------|---------|---------|---------|--------|-----<AAAAA-----------
        ABEL                                                        BAKER

And some sort of signal is required to indicate the status of the blocks:


WEST ABEL    EAST ABEL                                     WEST BAKER     EAST BAKER
      ---------                                                   -<BBBBB---
     /         \     G--#|     G--#|     G--#|     G--#|         /          \       
----|-----------|--------|---------|---------|---------|--------|-----<AAAAA-----------

For the purposes of this page I will ignore any sort of control that may or may not exist at ABEL or BAKER; in fact, I'll expand the map so that they hardly appear. I'll also ignore any signals in the opposite (eastward) direction.

Let's now say that Train A is given permission to proceed westward, and it enters the main track:


EAST ABEL                                                                 WEST BAKER
----             181            193            205            217               ----
    \          G--#|          G--#|          G--#|          G--#|              /
-----|-------------|--------------|--------------|--------------|---------<AAAAA----

On approaching signal 217, the train will see a green light:


EAST ABEL                                                                 WEST BAKER
----             181            193            205            217               ----
    \          G--#|          G--#|          G--#|          G--#|              /
-----|-------------|--------------|--------------|--------------|<AAAAA-------------

Signal 217 governs entry into the 217T block. It shows green because both the 205T and the 217T blocks are clear of trains. Train A accepts the green signal and proceeds past the 217 signal. The 217T track circuit will now detect the presence of the train and cause the 217 signal to "drop" to red:


EAST ABEL                                                                 WEST BAKER
----             181            193            205            217               ----
    \          G--#|          G--#|   205T   G--#|   217T   R--#|              /
-----|-------------|--------------|--------------|-----------<AAAAA-----------------

As train A proceeds through the 217T block, the 217 signal continues to show red:


EAST ABEL                                                                 WEST BAKER
----             181            193            205            217               ----
    \          G--#|          G--#|   205T   G--#|   217T   R--#|              /
-----|-------------|--------------|--------------|-----<AAAAA---|-------------------

When train A accepts the 205 signal and crosses into the 205T block, 205 will drop to red as well. Note, however, that since train A's tail is still in the previous block, 217 continues to display red:


EAST ABEL                                                                 WEST BAKER
----             181            193            205            217               ----
    \          G--#|          G--#|   205T   R--#|   217T   R--#|              /
-----|-------------|--------------|-----------<AAAAA------------|-------------------

Only after train A clears the 217T block entirely will the 217 signal change to another aspect. In this case, 217 will change to yellow or Approach:


EAST ABEL                                                                 WEST BAKER
----             181            193            205            217               ----
    \          G--#|   193T   G--#|   205T   R--#|   217T   Y--#|              /
-----|-------------|--------------|------<AAAAA--|--------------|-------------------

The Approach aspect warns the engineer that only one block ahead is clear, as opposed to the Clear (green) aspect, which indicates that there are at least two clear blocks ahead.

The next change occurs when train A crosses the next block boundary into block 193T:


EAST ABEL                                                                 WEST BAKER
----             181            193            205            217               ----
    \          G--#|   193T   R--#|   205T   R--#|   217T   Y--#|              /
-----|-------------|-----------<AAAAA------------|--------------|-------------------

After the train is entirely into the 193T block, the 205 signal will change to yellow:


EAST ABEL                                                                 WEST BAKER
----             181            193            205            217               ----
    \          G--#|   193T   R--#|   205T   Y--#|   217T   Y--#|              /
-----|-------------|---<AAAAA-----|--------------|--------------|-------------------

And very shortly thereafter, once the information gets passed back to signal 217's control circuitry, 217 will change to green:


EAST ABEL                                                                 WEST BAKER
----             181            193            205            217               ----
    \    181T  G--#|   193T   R--#|   205T   Y--#|   217T   G--#|              /
-----|-------------|---<AAAAA-----|--------------|--------------|-------------------

When train A passes entirely into block 181T, 193 and 205 will change, but 217 will not.


Effect of ABS on Train Density

Recall that previously, under totally manual control, only one train was permitted between stations (except under very strict conditions):


      ---------                                                   -<BBBBB---
     /         \                                                 /          \       
---------------------------------------------<AAAAA------------------------------------
        ABEL                                                        BAKER

Now with ABS, soon after train A leaves BAKER:


WEST ABEL    EAST ABEL                                     WEST BAKER     EAST BAKER
      ---------                                                   -<BBBBB---
     /         \     G--#|     G--#|     G--#|     R--#|         /          \
----|-----------|--------|---------|---------|--<AAAAA-|--------|----------------------

Train B may leave soon thereafter:


WEST ABEL    EAST ABEL                                     WEST BAKER     EAST BAKER
      ---------        181       193       205       217          BBB-------
     /         \     G--#|     G--#|     G--#|     R--#|         B          \
----|-----------|--------|---------|---------|<AAAAA---|------<BB----------------------

By the time train B reaches signal 217, train A may already have passed the 205 signal, and thus train B will see an Approach (yellow) aspect:


WEST ABEL    EAST ABEL                                     WEST BAKER     EAST BAKER
      ---------        181       193       205       217          ----------
     /         \     G--#|     G--#|     R--#|     Y--#|         /          \
----|-----------|--------|---------|--<AAAAA-|---------|<BBBBB-------------------------

Under most rule systems, train B would be required to proceed at a moderate speed, whereas train A would be allowed to proceed at full speed. Thus, they would tend to get farther apart, and the tendency would be for train B eventually to get a green light:


WEST ABEL    EAST ABEL                                     WEST BAKER     EAST BAKER
      ---------        181       193       205       217          ----------
     /         \     G--#|     R--#|     Y--#|     R--#|         /          \
----|-----------|--------|<AAAAA---|---------|-----<BBBBB------------------------------

WEST ABEL    EAST ABEL                                     WEST BAKER     EAST BAKER
      ---------        181       193       205       217          ----------
     /         \     R--#|     Y--#|     G--#|     R--#|         /          \ 
----|-----------|<AAAAA--|---------|---------|<BBBBB---|-------------------------------

Note that we can now easily fit 2 trains, and possibly more, on a line that previously could hold only one. This is the (safe) increase in train density that a signalling system allows.


Permissive Signals

I've not yet mentioned the reason for the '#' symbol on the signals shown above. This stands for the number plate on the signal and is an indicator that the signal is a permissive signal, and that it is not an absolute signal.

The purpose of the automatic signals 181 through 217 is merely to space the trains apart, not to hold them at a particular location. In fact, the main track between EAST ABEL and WEST BAKER may not have any switches along it anywhere. Or, there may be switches in the main track, but the spurs are too short to allow A to pull in and B to run around A. There may or may not be passenger stations between ABEL and BAKER, but even if there are, these stations may not have any passing sidings [crossing loops] (quite common in the US). In other words, there is no real reason for controlling a train between ABEL and BAKER, nor is there any reason to do so. With no reason to do so, there is no reason why any train should have to stop and wait for a signal to clear. Thus was born the permissive signal.

Railroads vary in terms of the degree of vigor they apply to such signals. Probably most common, or at least historically the most common, was the "stop and proceed". When implemented in this way, a full stop was required, sometimes with a brief waiting period, before the train was permitted to proceed at a slow, restricted speed. The severest known to the author was/is Amtrak, which requires a 3-minute stop of all freight trains. Other railroads decided that such vigor is not needed, and that a "Restricted Proceed" will do. The engineer's duties were the same; only the initial stop was eliminated.

By "restricted speed" is meant that the engineer had to be prepared to stop before any obstruction of the track, not the least of which was a train, or part of a train, in front of him. Other potential causes of a red signal include switches not properly lined for the main track and broken rail. Speed was restricted to 15 MPH (most commonly). Yes, the train was crawling, but at least it was moving. Chances were good that the red signal was caused by another train, and when out of the way, a better aspect would be received at the next signal.

Identifying a Permissive Signal

Most railroads identified a permissive signal by attaching a number plate to the mast or otherwise near the signal. If there were multiple signals, as on a bracket post or signal bridge, every permissive signal had to have its own number plate, lest the others be interpreted as absolute signals. (And yes, such cases did exist.)

Many railroads using color light signals staggered the heads of multi-head permissive signals. E.g., the uppermost head was mounted to the left of the signal mast, the middle head to the right, and the lowermost head (where present) again to the left. Where implemented, staggering was generally used in addition to the number plate to indicate a permissive signal. [Does CP and/or CN use staggering alone?] The author interprets this as meaning that the staggering was a visual aid to identifying a permissive signal at a distance, when the number plate could not yet be seen.

Grade Markers

Those railroads that required a stop recognized that some very heavy trains may not be able to restart on an uphill grade if they were required to stop for a red signal. Signals on such uphill grades would thus be equipped with a marker plate to indicate that the stop was not needed at that signal. The marker plate used varies from railroad to railroad; many used a "G" marker (for "Grade"; others used "P" for "permissive".

Some railroads allow the grade marker to apply to all trains; others allow it to apply to freight trains only. Previously, some railroads' rules applied the grade marker only to certain (particularly heavy) classes of trains.


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Automatic Block System


Mark D. Bej, M.D.
bejm@ccfadm.eeg.ccf.org
+216-444-0119
1997.05.09