Interlocking tower

On a rail transport system, signalling control is the process by which control is exercised over train movements by way of railway signals and block systems to ensure that trains operate safely, over the correct route and to the proper timetable. Signalling control was originally exercised via a decentralised network of control points that were known by a variety of names including signal box (International and British), interlocking tower (North America), signal poste (France) and signal cabin (some railways e.g., GCR). Currently these decentralised systems are being consolidated into wide scale signalling centres or dispatch offices. Whatever the form, signalling control provides an interface between the human signal operator and the lineside signalling equipment. The technical apparatus used to control switches (points), signals and block systems is called interlocking.

Originally, all signalling was done by 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 realised 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 and Croydon Railway in 1843 to control the junction to Bricklayers' Arms in London.

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