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 HISTORY OF THE GREENBANK SEMAPHORE
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.