Railroad Signals, Safely Keeping Trains On The Move
Railroad signals are an integral part of railroading, without them there is absolutely no way trains could safely operate, particularly on single track lines. However, more than just for safety signals allow for the efficient and steady flow of trains, enabling several to operate over a single-tracked line. Signals have their beginnings dating back to the industry's infancy itself with the famed ball-type signal. Perhaps, though, the most famous signal-type
of all was the semaphore, which came into use in the mid-19th century
and even today can still be found in regular service! While signal displays and meanings often varied from railroad to railroad some of the most common types were the color-light signals (or tri-lights), searchlights, position-lights, and color-position lights with the tri-light the most popular today.
The first railroad signals
came into use in 1832 on the New Castle & Frenchtown Railroad but
really came of age in 1872 with the invention of the track circuit by
William Robinson. This circuitry allowed for the steel rail's natural conductivity to be employed thus giving birth to automatic block signaling systems, the leader of which remains Union Switch & Signal created by George Westinghouse in 1881. Today US&S is part of Ansaldo STS, an Italian company. The NC&F's original signaling system, way before electricity itself,
used a ball (hence its name, the ball-type) that operated on a vertical
pole and pulley system with the high, center and low position
displaying various means (with the ball itself also usually both black
and white coloring). If the ball was high this was a clear-to-proceed
display giving way to the now common railroad term "highball".
Naturally, a the ball system required an operator at each location thus
proving to be a rather expensive operation.
electrified railroad signaling system was the semaphore . When the
semaphore came into use in the 1860s it was still an unpowered system .
Basically the semaphore worked
on a blade system whereby the indication was dependent on the position
of the blade. During the semaphore's infancy having proper sight of the
signal during the nighttime hours proved, obviously to be a problem
so later versions were equipped with red, yellow and green lenses so
that railroad engineers
could clearly see the indication being given. The earliest semaphores
known as lower-quadrant semaphores operated below the horizontal
position with a downward-vertical blade (on about a 60 degree radius)
meant proceed while a horizontal blade meant stop.
The most common semaphore type was the three-position, upper-quadrant signal
that was introduced around the beginning of the 20th century. It
operated above the horizontal position with an upward-vertical blade
meaning proceed, horizontal blade meaning stop, and a 45-degree position
meaning caution. This type of semaphore can amazing still be found in
use today, over 100 years since it became the standard semaphore type in
1908 (although be sure and get your photos of these structures while you can, many railroads are rapidly replacing them).
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,
Related Reading You May Enjoy
Share Your Thoughts
Have your say about what you just read! Leave me a comment in the box below. Please note that while I strive to present the information as accurately as possible I am aware that there may be errors. If you have potential corrections the help is greatly appreciated.