Centralized traffic control

Centralized traffic control (CTC) is a form of railway signalling that originated in North America. CTC consolidates train routing decisions that were previously carried out by local signal operators or the train crews themselves. The system consists of a centralized train dispatcher's office that controls railroad interlockings and traffic flows in portions of the rail system designated as CTC territory. One hallmark of CTC is a control panel with a graphical depiction of the railroad. On this panel the dispatcher can keep track of trains' locations across the territory that the dispatcher controls. Larger railroads may have multiple dispatcher's offices and even multiple dispatchers for each operating division. These offices are usually located near the busiest yards or stations, and their operational qualities can be compared to air traffic towers.

Key to the concept of CTC is the notion of traffic control as it applies to railroads. Trains moving in opposite directions on the same track cannot pass each other without special infrastructure such as sidings and switches that allow one of the trains to move out of the way. Initially the only two ways for trains to arrange such interactions was to somehow arrange it in advance or provide a communications link between the authority for train movements (the dispatcher) and the trains themselves. These two mechanisms for control would be formalized by railroad companies in a set of procedures called train order operation, which was later partly automated through use of Automatic Block Signals (ABS).

The starting point of each system was the railroad timetable that would form the advanced routing plan for train movements. Trains following the timetable would know when to take sidings, switch tracks and which route to take at junctions. However if train movements did not go as planned the timetable would then fail to represent reality, and attempting to follow the printed schedule could lead to routing errors or even accidents. This was especially common on single-track lines that comprised the majority of railroad route miles in North America. Pre-defined "meets" could lead to large delays if either train failed to show up, or worse, an "extra" train not listed in the timetable could suffer a head-on collision with another train that did not expect it.

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