A Newsletter of the White River Valley Museum

July 2001

Railroad Semaphore Signals

by Dr. Tina Brewster Wray, Curator of Collections



General Railway Signal Company
Model 2A semaphore signal.
The mechanism which operates the arm
is housed in the base.
Drawn by Rick Leach.

The White River Valley Museum is adding a working 1920's railroad semaphore to its permanent exhibits.

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