RE: (erielack) Erie Signals

[Gordon Davids <g.davids_@_verizon.net>]

For a short story, the reason for two arms (the correct term for each 
element, regardless of whether it is a semaphore or some form of a light 
signal) is that at least one of the aspects displayed by the signal 
requires two arms, as in "Approach Medium," (Yellow over Green).

For the long story, a signal that displays only three aspects using 
Green, Yellow or Red, one color at a time, requires only one arm.  The 
indication conveyed by a single Red aspect is further distinguished as 
either "Stop" (Rule 292) or "Stop and Proceed" (Rule 291) by the absence 
(Stop) or presence (Stop and Proceed) of a number plate on the mast.  As 
Steve Timko notes, the indication can be further modified by the 
presence of a "G" plate on the mast, which permits designated tonnage 
trains to proceed at a "Stop and Proceed" signal without having to stop, 
and the Erie further complicated things with Telephone Train Order 
signals, which is a whole 'nother topic.

In the case of a signal that displays four or more aspects, two arms (or 
three) are necessary.  If the most restricting indication conveyed by 
the signal is "Stop and Proceed," the arms will be staggered from the 
vertical position, and the mast will still hold a number plate.  If the 
most restricting indication is "Stop," the arms will be vertical and the 
mast will not hold a number plate.  That practice holds true on both 
single and multiple track.

In Erie practice, much of the automatic block and TCS territory used 
four-indication signaling, where the progression of indications on these 
intermediate signals was "Clear," (Green over Red); "Approach Medium," 
(Yellow over Green); "Approach," (Yellow over Red); and "Stop and 
Proceed," (Red over Red).  "Approach Medium" had a dual purpose.  It 
could govern the approach to any signal indicating "Approach" or to an 
interlocking home signal indicating "Clear Medium" or "Medium Approach."

Just as another technical note, besides the fine point of the term 
"arm," a signal "aspect" is the appearance of the signal as viewed from 
the rear (the direction from which a train is approaching.  A signal 
"indication" is the information conveyed by the aspect, such as "Stop, 
then proceed at restricted speed."  Each indication has a rule number 
(Rule 291) and a name, which is "Stop and Proceed" in this instance.  
One indication may have several different aspects associated with it, 
depending on the configuration of an individual signal.  For instance, 
"Approach" is a top Yellow with all lower arms Red regardless of how 
many arms are installed on the signal.  But every individual aspect may 
be associated with only one indication on any railroad, so there can be 
no ambiguity over the information conveyed by any signal.

Gordon Davids
> > Rich,
> > 
> > Many of the signal masts on the Erie NY Division were two 
> > searchlight heads, the upper being on the right side of mast 
> > and the lower on the left. As far as I can tell, these were 
> > located on double or quadruple track at locations not having 
> > Xovers, sidings or interlockings. What was the purpose of the 
> > lower head?
> > 
> > Paul B 
>   


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