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During the nineteenth century, as train speed and frequency increased, the
need for improved trackside traffic control became evident. The earliest
systems for keeping trains apart used signalmen located at stations (depots)
along the line, who used hand signals to indicate the state of the track
ahead. After 1869 with the inception of telegraph dispatching, communication
between signalmen was improved by the use of Morse code, but this
arrangement was labor-intensive and subject to human error, resulting in
excessive delays and sometimes, accidents. In 1871, the first automatic
signal was developed, and by the 1920's automatic signal systems (known as
Automatic Block Signaling systems or ABS) had been installed on the
mainlines of most railroads.
These ABS systems consisted of a series of semaphore signals mounted
along the track at various "blocks," or intervals (the earliest
ones being one mile apart). These semaphore signals consisted of a pole (or
mast) on which was mounted a movable arm (spectacle casing) with a wooden
blade. The angles of the arm variously indicated "stop"
(perpendicular to the mast), "clear/proceed" (vertical), and
"prepare to stop at the next signal" (a 45-degree angle). For
visibility at night, the spectacle portion of the blade contained colored
lenses that passed over a light source, displaying -- depending on the
respective arm angles, -- red, yellow, or green.
Movement of the arm was regulated by a motor-driven mechanism, powered
through a relay that was sensitive to an energizing electric current that
ran through the rails. As long as the current ran through the rails, the
relay fed power to the semaphore and the signal arm remained in the vertical
or clear position. However, when a train entered that section of track,
available current for the electronic relay was shorted out by metal wheels
across the rails, and the arm would drop to the "stop" position.
Through this system, information was communicated automatically to the
engineer as to the condition of the track ahead, thus facilitating the
safety and efficiency of train movement. This basic system remains in use
today -- but semaphore signals with their colorful, moving blades have
discontinued and replaced with color light units similar to traffic signals.
A little over a year ago, a 1920s General Railway Signal Company Model 2A
semaphore signal was donated to the museum by Dean Lawthers. Museum member
Dave Sprau (a rail historian and retired BNSF train dispatcher) has been
working long hours to refurbish and restore it to working condition. It will
be placed next to the caboose as an exciting new addition to our railroad
exhibits. Look forward to seeing it in operation when the museum re-opens.
Dr. Tina Brewster Wray
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