North American Signaling:

Chicago, Milwaukee, St. Paul & Pacific Railroad Absolute Permissive Block signaling

Published December 17 2021. By Carsten S. Lundsten.
Thanks to Michael Prader for relaying the source material to this document.


Between 1914 and 1916 the Chicago, Milwaukee & St. Paul Railroad electrified the 440 miles of mainline between Harlowton MT and Avery ID using 3000 volt Direct Current power. The CM&StP later became part of the Chicago, Milwaukee, St. Paul and Pacific, also known as the Milwaukee Road. Along wih this electrification the CM&StP installed a new Automatic Block Signaling (ABS) system over most of this mileage. the components were bought from the Union Switch and Signal Company of Swissvale PA while the engineering and construction was performed by the CM&StP's own signal forces. In order to be compatible with the Direct Current electrification the ABS used Alternating Current (AC) track and control circuits.

At the time the GRS (General Railway Signal Company of Rochester NY) Absolute Permissive Block signaling (APB) system was introduced and became popular with railroads to the point of later becoming more or less the standard ABS concept on single track lines. The CM&StP ABS seems in some respects to be influenced by the APB ideas but patent restrictions had prevented the US&S from delivering a similar system.

In the late 1930s, however, the CM&StP had become the CMStP&P, and started rebuilding the ABS into a true APB system, albeit still based on AC track and control circuits. When the APB-conversions started - around 1936 - the CMStP&P still used red, green and white signal aspects for "Stop (then Proceed)", "Approach" and "Clear"(rather than the later universal colorlight signal aspects of red, yellow and green). This was changed just a few years later, and only the red/yellow/green aspects are shown in this document. Some APB conversions took place in the late 1940s but the total timeframe is not known. Besides this rebuilt APB for DC electrified territories then CMStP&P also had more conventional APB on non-electrified lines.

The reader is assumed to be familiar with the Absolute Permissive Block signaling systems, as the basic concepts of the APB are not explained in this document.

Signal aspects and indications

The CMStP&P light signal aspects and indications relevant for APB were:

Signal Aspect MILW Rule Name Indication
501-A Stop Signal Stop
501-AA Stop and Proceed Signal Stop, then proceed at restricted speed
501-B Approach Signal Proceed preparing to stop at next signal. Train exceeding medium speed must at once reduce to that speed.
501-C Clear Signal Proceed
Source: The Consolidated Code of Transportation Rules and General Instructions, Edition of 1939

Typical configurations

In its default state, i.e. with no trains nearby, an APB-converted ABS line on the CMStP&P typically looked like this (the signal numbering is an example and not from the actual systems):

The layout of the line was the same as the original CMStP&P ABS, and differed from a standard APB line in that the intermediate block signals were not installed in back-to-back pairs, but rather alternated at each track circuit joint. Normal track circuit length was around 1.2 miles but could vary depending on local conditions. The length of sidings was typically around 1 mile. The number of track circuits and signals between sidings could vary with the distance between sidings. The configuration shown above was the most common - with sidings approximately 7 miles apart.

At both ends of a siding were - as with most ABS/APB systems - signals for both directions, of which one was the "Headblock" signal for the line towards next siding. Actually the conversion to headblock signals (removal of the number plates and thus converting them into absolute signals) of signals 36.7, 42.8 and 43.7 was the only external giveaway that the line was converted from ABS to APB.

When sidings were closer than the approximately 7 miles shown above, the number of intermediate block signals could be reduced and sometimes the track circuits on line were also somewhat shorter. As a minimum requirement the distance between signals 57.7 and 58.8 (and between 61.9 and 63.0) would need to be at least the combined stopping distances of an eastbound and a westbound train at maximum permitted speed:

Similarly, when sidings were spaced farther apart, an extra set of intermediate signals could be added:

The Chicago, Milwaukee & St. Paul APB example

To illustrate the workings of the Milwaukee Road APB system we will look at how a meet could take place with the APB. An eastbound train is to take siding and wait for a superior westbound at the siding at MP43. First the eastbound approaches the siding, with opposing signals 44-9 and 43-7 tumbled down to "Stop and Proceed" and "Stop" respectively. And signal 42-7 to "Approach":

When the eastbound approaches the siding signal 42-7 drops to "Stop and Proceed" and signal 40-1 consequently drops to "Approach":

Standard APB, as well as newer implementations of ABS, would have had signal 40-1 show "Approach" already when 42-7 dropped to "Approach". This feature leading up to a siding was provided in order to give additional warning for opposing trains coming up to a meet more or less simultaneously. This feature became known as "Double Yellow". The absence of "double yellow" is discussed later in this document.

The eastbound pulls up to signal 43-8 for the crew to hand-throw the switch into the siding. Due to a safety function in the electric circuits signal 44-9 stays at "Stop and Proceed" until the train has left signal 46.2's block:

Opening the siding switch drops both signals protecting the switch to red. Signal 42-7 is already red from the train but signal 43-8 drops to red when the switch is opened:

The eastbound train now pulls into the siding and off the main track. Since the sidings are not main tracks and are not track circuited the train is essentially gone from the ABS system. The ABS cannot tell if a siding is occupied or not, and the following sequence of events would be the same without the eastbound train in the siding. Note that, for the sake of the example, signal 36-8 by the next siding east has dropped to "Approach" as the first sign that the westbound train is approaching:

As the eastbound train crew restores the switch to normal position the signals around the siding clear up. And the westbound train moves closer:

As the westbound train leaves the siding area to the east, the eastbound signals until the next siding tumbles down to red:

The train drops signal 37.9 to red. After leaving the first westbound block headblock signal 36.7 changes to "Approach":

The train then drops signal 40.1 to red. After leaving the second westbound block signal 37.9 changes to "Approach" and 36.7 to "Clear". Note that signal 39.0 stays red until signal 37.9 changes to "Approach". This is for technical, rather than operational, reasons, in order to check proper functioning of some vital circuits in signal 37.9:

Finally the meet takes place. Headblock signal 42-8 goes back to "Clear" and the eastbound can continue east now if it has movement authority to do so:

With signal 42-7 clear - and the westbound train having just gone by - the eastbound crew can assure themselves that no other trains are approaching and they can open the siding switch for their train to pull out onto the main again:

Restoring the switch allows signal 42-7 to clear:

And both trains are on their way again:


Stations/Siding area functionality was not changed between the original Automatic Block Signaling (ABS) system and the APB rebuild. The signals protecting the siding area (signals 42.7 and 43.8 in the example below) had their controls overlapping into the first track circuit on the line. The approach signals (40.1 and 46.2), however, lacked the standard ABS/APB "double yellow" feature. Around 1943 the double yellow seems to have made its way into the Chicago, Milwaukee & St. Paul APB circuits, but only where "needed". It is not clear to the author why the "double yellow" was omitted in the first place or what was put in its place (likely some dedicated rule or local instruction regarding the approach to siding areas. The control lines around sidings for red looked like this:

To illustrate the consequences of the missing "double yellow" consider the following scenario, where two trains are approaching each other at speed due to some sort of movement authority error: A westbound train is approaching a siding expecting to continue without stopping. The train accepts signal 40-1 at "Clear" and keeps on going without slowing down. What the westbound crew cannot see yet is that an eastbound train by mistake (either by the train crew, a station agent or the dispatcher) did not wait for a meet further west but instead is approaching the siding from the other side (signal 43-7 at "Stop and Proceed"; signal 42-7 at "Approach"):

The eastbound train accepts signal 46-2 at "Clear" just before the westbound hits the control limits of signal 43-8, which would have set 43-8 to "Stop and Proceed" and 46-2 to "Approach". Both trains therefore continue at speed:

Continuing on, the westbound may see and accept signal 42-7 at "Approach" just around the same time as the eastbound would hit the control limits of it and replace it to "Stop and Proceed". The westbound would then slow down expecting to stop at signal 43-7 while the eastbound continues at speed:

Eventually the eastbound finds signal 43-8 at red but may not be able to stop before the signal and continues into the block that the westbound occupies (though the westbound is reducing speed). A collision is thus possible in this scenario:

In reality a number of factors would probably "save the day", like the westbound unexpectedly finding signal 42-7 at "Approach", possibly realize that something is wrong and slow their train even further. Or the eastbound train being able to see signal 43-8 at a distance and possibly be able to stop at or near the signal. The fact that the CMStP&P ABS was rebuilt into APB, and still did not adopt the "double yellow" feature, suggests that it was a very conscious choice, as the "double yellow" would be very simple to implement:

Comments, corrections and more information about ABS/APB systems are very welcome. Please use this contact form

[Back]          [Home]

Text, HTML: Carsten S. Lundsten.