North American Signaling:

Chicago, Milwaukee & St. Paul Railroad Automatic Block Signaling

Published January 14 2020 (minor corrections December 17 2021). By Carsten S. Lundsten.
Thanks to Jon Roma for providing the source material and for suggestions to this document and to Dave Sprau for historic corrections.


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 line was already signaled at the time but with semaphore signals and using standard direct current track circuits. Semaphores present visibility problems between the trolley poles and the Direct Current (DC) track circuits were not immune to the DC traction current in the rails. It was therefore decided to install a new automatic block signal system using light signals and using Alternating Current (AC) track and control circuits.

The project was described in the "Railway Signal Engineer" September 1917 issue and this document is based upon that article. According to the article 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. It is a fair assumption that the US&S has had the major role in the overall design of the system principles, and in writing the article, though the article does not state so.

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 likely patent restrictions may have prevented the US&S from delivering a similar system.

When the ABS was installed the CM&StP 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. The CM&StP signals additionally carried a red marker light on their masts as shown in the table below. The marker lights were a common practice among railroads at the time to mark signals as permissive (marker light to the left) or absolute (marker light to the right). From 1939 the CM&StP, which bty now has become the CMStP&P, used "The Consolidated Code of Transportation Rules and General Instructions" as its rulebook, and the signal conversion to red, yellow and green signal aspects and removal of marker lights were done at this time. From 1936 and onwards the signal system was converted into an APB type of block system. Some APB conversions took place in the late 1940s but the total timeframe is not known.

Signal aspects and indications

The CM&StP light signal aspects and indications relevant for ABS were:

Signal Aspect MILW Rule Name Indication
501-AA Stop and Proceed Signal Stop, then proceed
501-B Approach Signal Approach next signal Prepared to Stop
501-C Clear Signal Proceed

Typical configurations

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

The line was divided into more or less equal length track circuits. Normal length was around 1.2 miles but could vary depending on local conditions. The length of stations/sidings *) was typically around 1 mile. At both ends of a siding were signals for both directions while other locations only had a signal for one direction. 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.

*) The "Railway Signal Engineer" article uses the term "Station" but a more common term when discussing ABS is "Siding".

All signals were "permissive" signals, i.e. they carried a number plate which made their most restictive indication "Stop, then proceed", rather that a "Stop" indication. The default aspects for the signals were "Clear Signal". Switches were manually operated.

As standard with single track ABS lines the signals do not convey movement authority for the trains. Trains were operated by TimeTable and Train Order (TT&TO) and the ABS merely supported these operations by

The figure below shows the control lines of each signal, i.e. how far the signal "looked ahead" in order to show a proceed aspect ("Approach" or "Clear"). For example, if any of the 3 track circuits between signal 42.8 and signal 39.0 were occupied, signal 42.8 would show a red light. Similarly, signal 41.4 would look at any of the following 4 track circuits (i.e. up to signal 36.8), though the dashed line indicates that for following moves (i.e. a train following another train in the same direction) it would only look 3 track circuits ahead. A signal not showing a proceed aspect (i.e. a signal showing "Stop, then Proceed") would always be preceded by a signal showing "Approach".

Example: If the track circuit between signals 40-1 and 41-4 was occupied then those two signals would be red. And so would signals 37-9 and 42-8 since their control lines also included the occupied track circuit. Signals 36-7 and 43-8 would then show the "Approach" indication:

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. With only one set of intermediate signals the control lines for the end-of-siding signals would extend all the way to the next siding, similarly to the protection an APB installation would provide:

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

The Chicago, Milwaukee & St. Paul ABS example

To illustrate the workings of the Milwaukee Road ABS system we will look at how a meet could take place with the ABS. 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 both at "Stop and Proceed" ahead for protection. And signal 42-7 at "Approach":

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

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. In signal systems where "Approach" were displayed as a yellow light 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:

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. >The following sequence of events would be the same without the eastbound. Note that in this 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 on the line ahead of it drops to red. In this particular configuration of signals the eastbound train would now see the first sign of the approaching westbound as signal 42-8 drops from "Clear" to "Approach". Had the sidings been farther apart (i.e. with more intermediate signals) the westbound would still not be visible at this point. In configurations with only one intermediate signal between stations signal 42-8 would drop directly from "Clear" to "Stop then Proceed" as the westbound left the siding area (i.e. passed signal 36-7):

When passing intermediate signal 39-0 the train drops signal 42-8 to red while other signals clear up behind it:

Intermediate signal 37-9 changes to "Approach". Compared to when signal 41-4 dropped to "Stop, then Proceed" this illustrates that the controls of said signals vary depending on the train's direction of travel:

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

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 again but with the westbound train still keeping signal 43-7 red, 42-7 will only change to "Approach" in this case:

And both trains are on their way again:


As other types of early ABS the Chicago, Milwaukee & St. Paul's system mainly protected against opposing traffic on the open line. Opposing trains having to approach each other between sidings is usually a rare situation (switching local industries, work trains) and the movement authorities would not be allowed to actually overlap except perhaps at Restricted Speed. In most cases trains opposing each other between sidings would be the result of a mistake, either in dispatching or by a train crew. But as long as the trains were obeying their signals they would meet each other at no more than Restricted Speed and thus a collision was avoided.

Obviously trains would regularly be approaching a siding from both sides for a meet. Having the siding entrance signals drop to red far ahead of the opposing train as on the open line would slow down meets considerably. While the ABS did ensure that opposing trains could not enter the same block by "Clear" or "Approach" signals, it did not in all cases provide enough warning to ensure that the trains could obey the "Stop and Proceed" signal they would encounter when coming up on an (unexpected) opposing train. Still, only one train should have the authority to occupy the main track at the siding area so two trains approaching each other at speed woulds be due to a mistake in the first place. The control lines around sidings looked like this:

Consider the following scenario: 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, then 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, then 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, then 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 low speed 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. Besides the fact that this scenario would require a fairly precise and unfortunate timning to materialize.

As mentioned earlier The CM&StP ABS was designed and built around the same time as GRS was building and marketing the Absolute Permissive Block signaling (APB) system, which in many respects was better than this design. The APB featured the so called "Double Yellow" principle which ensured that a train would encounter an extra "Approach" signal (using the later standard yellow for "Approach") before a siding when an opposing train is approaching:

Translated to the The Milwaukee Road ABS the scenario above would instead have started like this and both trains had plenty of early warning:

The CM&StP ABS was from 1936 and onwards rebuilt into an Absolute Permissive Block signaling functionality. The double yellow function was not added at this point, but was later included as an option for circuit designs.

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Text, HTML: Carsten S. Lundsten.