Semaphore signals were first developed for railroad use in England during the 1840s from earlier semaphore communication telegraphs. Once adapted for railroad use, they allowed information to be transmitted to the engineer and/or conductor of a moving train. A success, they crossed the Atlantic in the 1860s and dominated American railroad signaling from the 1890s through the 1930s. They supplanted earlier technologies involving flags, balls, vanes, banners and disks.

A semaphore consists of a pole or other support structure with one or more movable arms called "blades." The position of the blades conveys specific information to the train crew. For night, colored lenses or "roundels" held in "spectacles" attached to the blades are moved in front of a lantern. The shape and color of the blades and the presence of additional "marker" lights or signs on the pole provide additional information. Each combination of blade positions, colors, shapes, lights and markers is an "aspect" and the message it represents is an "indication."

The indication might refer to the presence of another train ahead, the speed or speeds the engineer should employ on upcoming track, the route expected through an upcoming series of switches, or the need to obtain train orders. Knowing all the possible semaphore signal aspects and what each indicated for a particular railroad was a required skill for engineers, firemen, conductors, and brakemen – just as it remains today with modern signal systems.

Originally, semaphores were entirely mechanical. An operator moved the blades by throwing "Armstrong" levers (so-called because strong arms were needed) which were connected to the blades by rods and cranks.

Many manufacturers and designs of semaphores existed. The most popular and widely used American electrical semaphore was the General Railway Signals Inc. (GRS) Model 2A, with over 25,000 installed. The standard Model 2A was an upper quadrant, 3-position semaphore, meaning it had three possible positions per blade: horizontal, diagonal up, and vertical up.

The Market Street Branch semaphore was a GRS Model 2A. According to GRS information, it was sold to the B&O between 1917 and 1919, but its full history with the B&O is unknown. At some point prior to the end of World War II, it was installed on the Market Street Branch and used to signal permission to proceed for trains leaving the West Yard for Elsmere Junction.

Junction-bound trains were forbidden to pass the signal at "Stop" – blade horizontal with a red light – under normal operation. The train crew would contact the junction operator by telephone or (later) by radio. After the route was lined and verified free of traffic, the operator would set the semaphore to "Clear" – blade vertical with a green light. When the train entered the block, the signal would be set back to "Stop." In later years, a dispatcher in Baltimore controlled the signal.

The Market Street Branch’s semaphore had an ignoble end to its service life with a mainline railroad. In the late 1980s, it ceased working due to neglect, and in the early 1990s its supporting pole collapsed. It was damaged by the fall and lay immersed in the water and mud of the trackside drainage ditch for at least a year. It was never replaced by CSX, the current owner and operator of trains on the Market Street Branch.

The work required to restore the nearly century-old semaphore was significant. A pole was found in a salvage yard near Cleveland, and a new ladder and platform assembly were fabricated using original plans. A nearly five ton concrete foundation was poured and set in place. Parts large and small were sand blasted and painted. The original spectacle and blade were repaired; proper colored glass roundels and a period-correct GRS lantern were found. A Block Signal Control Cabinet for Greenbank Station was built following original B&O plans, and more than a thousand feet of wire was run between the station and signal.