A semaphore was a trackside visual signal device that instructed the train engineer to: (a) stop, (b) proceed with reduced speed or (c) proceed at normal speed. It consisted of a movable arm (“blade”) mounted on tall trackside pole and included illuminated colored lights for night use.
Railroad lines are divided into a series of control sections called “blocks” where only one train may occupy a block at a time. Each block is long enough to allow a train to stop within them. This ensures that a train always has time to stop before encountering another train on the same track.
The semaphore system of signals was used to control flow between the blocks. When a train entered a block, signals at both ends changed to indicate that the block was occupied. Some systems were for single track railways to prevent both head-on and rear-end collisions, whereas double track systems needed to prevent rear-end collisions.
There were two basic forms of semaphores. If the blade moved downward from the horizontal, it was a lower quadrant semaphore. If the blade moved up from the horizontal position, it is called an upper quadrant semaphore.
The image shows an upper quadrant semaphore in its three positions: (a) horizontal/red = stop, (b) diagonal/yellow = caution, (c) vertical/green = proceed normally. Note there was a semaphore at Gibsonville Depot in the1920’s.
Semaphores were patented in 1840 and were the predominant control system until the 1940’s when trackside control lights replaced the swing arm semaphore that could jam in the wrong position and was hard to see. Today most track control information is provided electronically to the engineer and there are no more semaphores.