More on Interlocking signals

NW Mailing List nw-mailing-list at
Thu Jan 16 17:51:55 EST 2014

The "less favorable" indication is conveyed by the lower height of the arm
displaying what is otherwise an identical aspect and seems to apply only to
passing sidings. Signals governing the junction of secondary tracks such as
middle tracks, branches and some yard tracks typically use diverging
indications and without the height discrepancy when sharing a bracket or
bridge with the main signal. I asked about this inconsistent application of
aspects many years ago and got Jimmy's response. Train crews knew how to
interpret them, so safety was not an issue.

This likely dates back to semaphores, many mounted on these same masts,
indicating different routes by their relative height (rank), top to bottom:
main or high-speed route, medium-speed route, and low-speed route (speed
being relative). The siding's mast would be shorter for a semaphore arm at
the medium height indicating the lesser of the two routes. This was
two-block route signaling in its purest form applied to a CONverging
situation, so the indication was Clear, Approach, or Stop, depending on the
switch position and track conditions beyond. Position-light signals simply
replaced semaphores at the same medium height with the same indications
displayed by equivalent aspects. To my knowledge, N&W rules did not equate
the medium-ranked Clear or Approach in this case with Medium (Diverging)
Clear or Medium (Diverging) Approach even though the interpretations were
the same.

Grant Carpenter

> Date: Fri, 01 Nov 2013


> I would simply say that the rationale is in keeping with the status quo.

> Why change?


> Jimmy Lisle


> Date: Fri, 1 Nov 2013


> I have been a lurker here for a little bit and have some questions

regarding N&W signals and the conversion from CPL's to standard tri-light
safetran signals which has been happening for the last 15-20 years.


> First question is in regards to signals at the end of a siding. Most all

other railroads when approaching the end of a signaled siding would have a
dwarf on the siding track to the main or a regular signal mast which would
give a "less favorable" aspect than clear or approach. You would think in
N&W signal terms, when approaching the end of a signaled siding to a single
main, you would receive a "diverging" aspect of some kind, like diverging
clear, or diverging approach. In fact, some signal replacement projects did
exactly this. Where some just kept with the "N&W way" of allowing a clear
signal at the end of the siding to single main track.


> What was the rationale for N&W doing this back in the day


> What is the rationale for the way they are doing things now.


> -Troy Nolen

> Tampa, FLA.

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