Centralized traffic control (CTC) is a form of railway signalling that originated in North America and centralizes 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 CTC territory. One hallmark of CTC is a graphical depiction of the railroad on which 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 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 trains moving in opposite directions on the same track cannot pass each other without special infrastructure such a siding and switches that allow one 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 in what was known as Train order operation which was then partly automated through use of Automatic Block Signals.