One exception to signal face shape is the B&M, which used searchlight-style signal heads
(one lens assembly per head) for mainline signals and horizontally oriented 3-lamp color light
heads for train order signals.
Many railroads that used color-light signals distinguished automatic signals from
interlocking signals by staggering the arms. E.g., if the signal face on the upper arm
was to the left of the signal mast, the middle arm was to the mast's right, and the lower
arm to the left again. The left-right-left versus right-left-right pattern varied from
road to road and was inconsistent on some roads. In contrast to automatic signals,
interlocking signals on these roads were installed with all signal faces in a vertical
line.
Staggering appears to have been most commonly implemented with the upper arm to the right
of the signal mast, the middle arm to the left, and the lower arm (when present) again to the
right. At least one railroad (B&M) appears to have followed the opposite convention.
(Did other conventions exist, e.g., outside-inside-outside relative to centerline of double track?)
The position-light signal systems (PRR, B&O, N&W) never staggered the signal arms
and, consequently, never showed it in the rules.
Significance of number plate.
Number plates are significant on most railroads. These are used to identify an automatic
signal, and to distinguish it from an interlocking signal. The signal-numbering scheme varied
from road to road but was frequently based on the milepost. A common scheme was to use the
milepost, including tenths of a mile, times 10. Additional provisions had to be made for
multitrack or bidirectionally signalled lines; these varied markedly from road to road. On the
PRR, e.g., E/B signals were numbered even, W/B odd, with one of the signal numbers bearing the
number of the next nearest tenth of a mile (though which was the larger number was
inconsistent). On signal and track diagrams, but not on the signal itself, the track was denoted
after a hyphen, e.g. "178-2" was the eastbound (even) signal near milepost 17.8 on track 2. Conrail
uses a scheme based on the full milepost, with the track number and "E" or "W". That same signal
would be numbered 182E, i.e., milepost 18, track 2, eastbound.
Significance conflicts
For a number of roads which used number plates and staggered signal arms, it is not at all
clear from the rules which of these 2 items is, or if both are, significant.
Marker lamps, etc.
On some roads, additional lamps or other elements were used to underscore the signal's status
as an automatic signal. Examples are B&O and PRR. These marker lamps were redundant only at
automatic signals; at interlocking signals they could be used to change the meaning of the signal.
Some railroads placed train order signals near interlocking signals. These are generally
not considered part of the interlocking signal.
Other marker lamps were (and are) occasionally used, to indicate route, or for various other
purposes. Such additional marker lamps are generally shown in the rulebook under the heading
"Signals not in Conformity with Typical Aspects" or some substantially similar statement.
Examples include route indicators on the Northeast Corridor; a 3-amber-light N&W signal
which never got "updated" to the colorized, 2-lamp variety; square-headed signals at Kiski
Jct. near Pittsburgh; among others.
Vertical placement of signals relative to one another
Some railroads denoted a signal governing a spur or siding by placing it lower with respect
to the signal(s) governing the main track(s). On some railroads, notably N&W, this appears
to have been a sanctioned and possibly consistent practice. How "official" this was remains to
be determined. No rulebook example has been found as yet.
Assumptions made in signal diagrams in books of rules
- Number plate
- Many railroads showed the number plate in their rulebook only when it made a difference
in the interpretation of the rule. Examples include PRR, NORAC. Others noted, in text, that
the presence of absence of a number plate does not change the meaning of the rule unless the
diagram shows the number plate (e.g. BN). Still others showed both forms in all cases (e.g.,
SOU), and then frequenly showed the arms staggered.
- Staggering of arms
- The philosophy on whether to show this in the rules varied as well. As mentioned, SOU
specifically showed staggered and aligned forms of its signals, even when the color sequence
was the same. Other railroads showed only nonstaggered forms, the staggered forms implicitly
bearing the same meaning -- this, despite staggering their automatic signals just as
consistently.
- Dark arm
- A dark signal arm is assumed in certain PRR and N&W signal rules. Dark marker lights
(but not the main arm) in B&O signalling are assumed. In the color-light systems, rulebooks
appear never to assume a dark arm, but rather, always to show a dark arm if such is intended.
How conflicting aspects are notated
Most railroads had at least some aspects that were either not noted in their rulebook
or aspects that conflicted with the standard aspects. These were shown, usually division by
division, in a special section of the Employee Timetable and Special Instructions. Some of these
notations were unusual installations (e.g., color lights in Pennsylvania Station, New York).
Some were older signal installations (semaphore or other signals). Some resulted from trackage
rights over another railroad.
Rule numbers
The AAR [American Association of Railroads] set forth a standard set of aspects and rule
numbers. Many U.S. railroads followed this rule numbering scheme, which generally proceeds from
the highest-speed aspects to the lowest-speed aspects. Additional aspects, probably added after
the AAR scheme was developed, were notated with a rule number and a letter. The latter lettering
scheme was not entirely consistent from road to road.