Position-lights were the chosen railroad signals for use on the Pennsylvania Railroad and Norfolk & Western Railway. The system used a standard yellow light usually with three lights
arranged in a semicircle to the upper right and three to the lower left
with one in the center. Depending on what type of indication the
three-aligned lenses were giving (i.e., vertical, horizontal or
diagonal) the train would either proceed, stop or some other cautionary
measure (such as stop and proceed). Position-light signals can still be
found on former PRR and N&W routes by current owner Norfolk
Southern is slowly replacing them with the redundant tri-light for easier maintenance.
The searchlight would become one of the most popular railroad signals during the classic era of the 20th century. It was introduced in 1920 and used as only one lamp that displayed a green, yellow and red indication by way of rotating color lenses.
It found use on dozens of classic railroads such as the Rock Island,
Union Pacific and Atlantic Coast Line but the high amount of maintenance
required to keep the colored lens system functioning has caused it to fall out of favor for the much easier maintained tri-light signal.
Color Position-Light Railroad Signals (CPLs)
position-light signals, or CPLs, are virtually synonymous with the
Baltimore & Ohio Railroad. The B&O's is a simplified version of
the PRR position-light in that it has no center lens
but essentially functions in the same way as the PRR version save that
it uses different colors to project its meaning and not the standard
yellow. For instance, two vertical green lens is a clear and proceed
indication whereby a yellow diagonal indicates caution and a red
horizontal means restricted (stop). The B&O also used a white light
above the signal for further instructions (sometimes cantilevered).
Color-light signals came into use around 1914 and are essentially an inverted highway stoplight with green above (proceed), centered-yellow (caution) and restricted below. However, other versions of this railroad signal include the triangle design, which operates basically in the same fashion just with a different layout. Also known as tri-lights they are, today, the favorite among Class I railroads with lines like CSX and Norfolk Southern Railway replacing their historic B&O and PRR versions with color-lights for ease of maintenance.
Railroad Signals, How They Function
signals, historically and even today are a lot more complicated, in
terms of their meaning than your basic highway stoplight. To begin
railroad signals serve to functions, protection and control and there
are two different types; permissive and absolute. A permissive signal
provides only protection within a given block (meaning a section of
track along a rail line having several blocks between Point A and Point
B, each protected by a set of signals) while an absolute signal
provides both protection and control (i.e., meaning it protects the
block while also giving a train ruling instructions). For instance,
when a permissive signal gives a red indication this means stop and
proceed while an absolute signal displaying a red indication means stop
and do not proceed without further dispatcher instructions.
After William Robinson's invention of the track circuit in 1872 this gave railroads the ability to signal blocks of rail lines. The first system was the Automatic Block Signal which used a masted-signal illuminating red, yellow or green to give an indication. It worked by using low-voltage batteries along the block and as long as the rails were clear of trains a green, proceed indication was shown but when a train entered the block it broke the circuit with both signals then displaying red, or restricted. Centralized Traffic Control, the most commonly used signaling system today uses both ABS and dispatchers to control the movement of trains as an extra measure of safety.
So, just because a signal
may display a green, proceed indication the train cannot move without
verbal permission from the dispatcher (unless it has already been given
permission beforehand to pass through the upcoming block). Other
signaling systems that were once or are still in use include Direct
Traffic Control (DTC), Form D Control System (DCS) and Track Warrrant
Control (TWC). Although it has been mentioned several times above
railroad signal displays (green, yellow, and red) mean three things.
First, aspect is what the signal
is displaying such as clear to proceed (green) or restricted to stop
(red); second, indication is the instruction the display is giving; and
finally third, the name of the display given (i.e., restricted, proceed,
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