Signal Question and rule term

NW Mailing List nw-mailing-list at
Tue Oct 29 11:17:04 EDT 2013

Brandon Kabak asks about the matter of names for N&W signal aspects.

I really should do an article on this sometime, but few people would read past the second paragraph.

Without getting into a lot of detail, suffice it to say that a railroad which (a) has high speeds and (b) has complex interlockings and (c) runs a lot of passenger trains on close headways and (d) has high-speed turnouts and crossovers, needs a more sophisticated signal system. If your railroad doesn't have any of these items, then you can get by much more cheaply with a farmer's signal system. By a "farmer's signal system" is meant one in which the signals do not tell the engineman anything about the speed requirements of the switches over he will be traveling, but only tell him he will be "diverging" and to proceed "at the prescribed speed" (which he must figure out from his Time Table.)

Thus, you see that there are two ways of signaling a railroad: Speed Signaling, or Route Signaling.

<< EXAMPLE: Suppose you have an engineman coming to a complex interlocking which has a crossover good for 40 MPH, turnouts good for 30 MPH, and turnouts good for only 15 MPH. A Speed Signaling system can tell the engineman exactly how to proceed. A Route Signaling system falls flat on its face. >>

So, was the N&W a "Speed Signaling" railroad, or a "Route Signaling" railroad? The answer is that the N&W briefly flirted with some of the concepts of Speed Signaling (beginning about 1951,) but never implemented the full advantages of Speed Signaling, preferring instead to stay with the basic concepts of Route Signaling. After the 1964 N&W/WAB/NKP "merger," the N&W gave up entirely on Speed Signaling and lapsed back to Route Signaling only.

The following information is based on the 1930, 1945 and 1951 N&W Rule Books.

None of the speeds which are usually associated with signal names (Normal, Medium and Slow Speeds) even appear in N&W rule books until 1945 (about 25 years after they had started appearing in the rule books of other railroads.) In the 1945 book, the term "Medium Speed" is used in connection with only one signal Aspect, the Approach. (Interestingly, the 1945 book does not even assign rule numbers to the signal aspects... that didn't come along until the 1951 rule book.)

The 1951 rule book employs, for the first time, the term "Medium" in the names of signal aspects: Approach Medium, Medium Clear and Medium Approach. What is most curious, however, is that the word "Medium" appears in the name of the signal aspects, but the concept of Medium Speed ("not exceeding 30 MPH") is entirely missing in the definitions given -- they only say "proceed at prescribed speed."

What this tells me is that the N&W probably didn't have any main track turnouts that were good for more than 30 MPH (which is called "Medium Speed" in signal vernacular.) And also that they may have used the Medium Clear and Medium Approach aspects to govern movement over main track turnouts rated for less than 30MPH (which may be the reason for the the N&W's ubiquitous clause "at prescribed speed.")

Railroads with more full-blown signal systems used the concept of Slow Speed ("not exceeding 15 MPH") in signaling as early as 1920, and the faster roads even used Limited Speed ("not exceeding 40/45 MPH") beginning in the late 1930s. The N&W never used Slow Speed or Limited Speed in its signal aspects/indications (e.g. Approach Slow, Slow Approach, Slow Clear, Approach Limited, Limited Clear.)

At the time of the acquisition of the Wabash and NYC&StL Railroads, the decision was made to eliminate all use of "speed names" (Medium, Slow, etc) from the signal indications. This was reflected in the blue rule book issued shortly after the "merger" in 1964, in which all indications lapsed back to simply "diverging" aspects. This decision is extremely curious to me, as both the WAB and the NKP were fast railroads and used Speed Signaling! I have never found anyone who could tell me why the decision was made to junk Speed Signaling.

The NS continues to operate the former Conrail routes with the Speed Signaling system which has been in place for 90 years, but the rest of their property is operated with Route Signaling ("diverging" signaling.)

If you want to study a really beautiful and sophisticated signal system, check out the B&O's Color Position Light signal system created by it's British Chief Signal Engineer, Frank P. Patenall, in the 1920s. It is a very progressive system and loaded with flexibilities which kept trains moving at the best possible speeds. In my humble and ill-informed opinion, it is the best signal system ever created.

-- abram burnett,
retired turnip farmer in pennsyltucky

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