Editing Signalling control (section)

== History ==
[[Image:Des Plaines interlocking tower.jpg|thumb|right|Signal box and tracks at Deval interlocking, [[Des Plaines]], in 1993]]
Originally, all signaling was done by [[machine|mechanical means]]. Points and signals were operated locally from individual levers or handles, requiring the signalman to walk between the various pieces of equipment to set them in the required position for each train that passed. Before long, it was realized that control should be concentrated into one building, which came to be known as a signal box. The signal box provided a dry, climate-controlled space for the complex interlocking mechanics and also the signalman. The raised design of most signal boxes (which gave rise to the term "tower" in North America) also provided the signalman with a good view of the railway under his control. The first use of a signal box was by the [[London & Croydon Railway]] in 1843 to control the junction to [[Bricklayers Arms railway station|Bricklayers Arms]] in London.<ref name="Turner">Turner, J. T. Howard London Brighton and South Coast Railway, Part 1, Batsford, 1977 pp. 196–8</ref>

With the practical development of electric power, the complexity of a signal box was no longer limited by the distance a mechanical lever could work a set of [[Railroad switch|points]] or a [[Railway semaphore signal|semaphore signal]] via a direct physical connection (or the space required by such connections). Power-operated switch points and signaling devices greatly expanded the territory that a single control point could operate from several hundred yards to several miles.<ref>Principles of Electric Locking by James Anderson</ref> As the technology of electric [[relay logic]] was developed, it no longer became necessary for signalmen to operate [[Lever frame|control devices]] with any sort of mechanical logic at all. With the jump to all electronic logic, physical presence was no longer needed and the individual control points could be consolidated to increase system efficiency.

Another advancement made possible by the replacement of mechanical control by all-electric systems was that the signalman's user interface could be enhanced to further improve productivity. The smaller size of electric toggles and push buttons put more functionality within reach of an individual signalman. Route-setting technology automated the setting of individual points and routes through busy junctions. Computerized video displays removed the physical interface altogether, replacing it with a [[point-and-click]] or [[touchscreen]] interface. Finally, the use of [[Integrated Electronic Control Centre|Automatic Route Setting]] removed the need for any human input at all as common train movements could be fully automated according to a schedule or other scripted logic.

Signal boxes also served as important communications hubs, connecting the disparate parts of a rail line and linking them together to allow the safe passage of trains. The first signaling systems were made possible by technology like the telegraph and [[British absolute block signaling |block instrument]] that allowed adjacent signal boxes to communicate the status of a section of track. Later, the telephone put centralized dispatchers in contact with distant signal boxes, and radio even allowed direct communication with the trains themselves. The ultimate ability for data to be transmitted over long distances has proven the demise of most local control signal boxes. Signalmen next to the track are no longer needed to serve as the eyes and ears of the signaling system. [[Track circuit]]s transmit train locations to distant control centers and data links allow direct manipulation of the points and signals.

While some railway systems have more signal boxes than others, most future signaling projects will result in increasing amounts of centralized control relegating the lineside signal box to niche or heritage applications.