In the earliest days of the railroads, trains ran up and down single tracks and signaling was not a necessity. As rail traffic and speeds increased, however, train safety grew to be a major concern and signaling became essential to prevent collisions. Hand and arm signals were soon replaced by flags and lanterns, and in 1832 the first elevated wayside signaling was introduced. By the 1860s, mechanical signals were in general use, but no single system was agreed upon. Semaphore signals were widely adopted in Britain, but were not standardized until 1923, while ball signals were commonin the US. Color light signals came into use from the 1950s.
A chief safety mechanism in train signaling outside the US is the block system, which allows only one train to enter each “block” of a railroad at a time. In the 19th century, tokens provided evidence that a block was free. In the original “staff and ticket” system, the tower operator gave the locomotive engineer a token or “staff” to allow entry to a block. At the other end, the staff was given up, allowing a train to proceed in the opposite direction. If a second train followed the first along the same “block,” both carried written permission or a “ticket.” Later systems were operated by means of tokens inserted in a trackside machine.
Early signal systems
As railroad networks became more complex, rail operators largely relied on scheduling to maintain train distances and prevent accidents. However, signals indicating if a line or “block” (section of the line) was clear were crucial in case of timetable alteration or train breakdown (the last three signals in the following are UK-specific).