The Evolution Of Railroad Signaling Systems

What type of signal lamps are commonly used in the railroad system today?

Without railroad signals, trains could not safely operate especially on single track lines. Aside from the safety factor, railroad signals allow for the steady and efficient flow of trains and at the same time enable several trains to operate over a single-tracked line.

First Signal: Ball-Type

Signals for railway systems date back from the industry’s infancy period with the ball-type signal which became famous at that time. But what can be considered as the most famous signal-type may be the semaphore which was used in the mid-19th century. Semaphore is still being used in some railway services in the country.

Railroad signal displays and meanings vary from system to system. Some of the most common types of railway signals include:

Specialty Bulb - Signal Lamps - Sept 2013 MILW_MB_HoffmanSemaphores. It was the first electrified signaling system used in railroads. In the 1860s when semaphore was introduced, it was an unpowered system. It only worked on a blade system where the position of the blade determined the indication. The early semaphore versions became a problem for proper sighting especially at nighttime so improvements were made on the later versions which were equipped with green, yellow and red lenses to allow the engineers to clearly see the signal indication being given.


Position-Light Signals. These were the railroad signals chosen for the Pennsylvania Railroad and Norfolk and Western Railway. This signal system used a standard yellow light with three lights positioned in a semi-circle to the upper right and three to the lower left and with one in the center. The three aligned lenses can give vertical, horizontal or diagonal indications which would allow the train to either stop, proceed, or stop and proceed. The Pennsylvania Railroad and the Norfolk and Western Railway still have some position-light signals installed and used in the system.Specialty Bulb - Signal Lamps - Sept 2013CRIP_MB_Rocket


Searchlight Signals. During the 20th century, searchlight signals became one of the most popular railroad signals. These railroad signal lamps were introduced in 1920 and were the only type that displayed a red, yellow, and green indication using a rotating color lenses. Searchlight signals were used in the Rock Island, Atlantic Coast Line and the Union Pacific. However, high maintenance costs had caused the development of the easily maintained Tri-light signals.

Color Position-Light Railroad Signals (CPLs). Mention this railroad signal and Baltimore and Ohio Railroad will surely be mentioned. The CPLs are simplified versions of the position-light signals of the Pennsylvania Railroad. Both railroad signals function in the same and the only difference is that the B&O railroad uses different colors to project its meaning. The position-lights use only standard yellow lights.

Specialty Bulb - Signal Lamps - Sept 2013Chessie_DK_ColorLight

Color-Light Signals. These came into use around 1914. They are essentially an inverted highway stoplight with green indication as “proceed”; and centered-yellow as “caution”. Other versions of this railroad signal include the triangle design operating in the same way on a different layout. These are known today as tri-highlights.

It should be understood though that even if the railroad signal displayed a green which indicated to proceed, the train cannot move without the verbal permission from the dispatcher.

Which type do you think is the most functional?