Some Technical Information On Position Signals

POSITION LIGHT SIGNALS (PL's)

PRR originally used semaphores but switched to electric light because they were less expensive to maintain.  So, "Position" = Semaphore position. "Light" = Electric Light.  Color lights were added later so "Color" modifies Light to indicate the use of red, first, then green and lunar which came later on. The monochromatic signals (which are “Anti Fog” yellow for gantry and mast signals and “Lunar” white for dwarf and pedestal signals) used by the Pennsylvania Railroad and its successor Conrail are correctly called “Position Light” aka “PL’s” With the introduction of red lights used on horizontal aspects, the correct term is “Position Color Light” aka “PCL’s”.

Position Color Light's (PCL's)

The Norfolk and Western used a mixture of both “Position Light” and “Position Color Light” signals. Amtrak’s North East Corridor Signals are also technically termed “Position Color Light” aka “PCL’s”.

COLOR POSITION LIGHT SIGNALS (CPL'S)

The signals developed and used by the Baltimore and Ohio Railroad are technically called “Color Position Light” aka “CPL’s”. B&O's signals are a bit more complex.  "Color" refers to the use of Red, Yellow, Green, and Lunar on the main head - each of which is significant in terms of how many blocks are clear.  "Position" refers to the 6 marker lights - each of which is significant in indicating Route and Speed.  "Light" indicates the use of electric lights instead of the older semaphore system.

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