Railroad signals were developed to let train crews know about conditions on the track ahead of them. Signaling systems became necessary in the 19th century when early steam-driven trains began to move so quickly that they ran the risk of colliding with one another. Smooth rails and wheels allowed trains to carry heavy loads easily and efficiently, but as speeds and load weights increased, trains needed longer stopping distances. Train crews had to be sure that they were not headed toward another train coming in the opposite direction on the same track, and they had to maintain a safe distance between trains moving in the same direction.
The first attempt to avoid accidents was the adoption of a timetable system. This system was based on running trains on timed schedules, so that there was always a space between them. However, if a train broke down, the next train's crew had to be informed somehow so that it could react appropriately. In 1837, on a rail line in England, a telegraph system was introduced in which signals were sent on telegraph wires between stations up and down the tracks. The track was divided into blocks, or sections, with a signalman responsible for each block. As trains passed through the blocks, one signalman telegraphed messages to the next block, allowing the next signalman to decide whether it was safe for the train to proceed through that block.
In 1841, a system was devised for communicating with train operators using a mechanical version of semaphore arm signals. At night, when the signal flags could not be seen, a light source was used, with different colored lenses that were rotated in front of it. In time, various codes and rules were developed so that train crews could be kept informed about track conditions ahead as they moved from block to block.
As rail traffic increased, many refinements in signaling systems reduced the chances of human error and helped make train traffic run more smoothly. In 1872, an automatic block system was introduced in which the track itself was part of an electrical circuit, and various signals were activated when the train passed over the track. A modern version of this invention is the moving block system, in which a kind of zone is electronically maintained around a train, and the speed of nearby trains is regulated automatically. Today, traffic control in rail systems is largely centralized and computerized. Many trains and cars can be monitored at one time, and signals and switches can be operated remotely to manage the system with maximum safety and efficiency.
In order for these sophisticated controls to be effective, railroad signals and signaling equipment must function properly. Signal mechanics ensure that this vital equipment is working as intended.