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. 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. Therefore, timetable operation was supplemented with train orders, which superseded the instructions in the timetable. From the 1850s until the middle of the twentieth century, train orders were telegraphed in Morse code by a dispatcher to a local station, where the orders would be written down on standardized forms and a copy provided to the train crew when they passed that station, directing them to take certain actions at various points ahead: for example, take a siding to meet another train, wait at a specified location for further instructions, run later than scheduled, or numerous other actions. The development of Direct Traffic Control via radio or telephone between dispatchers and train crews made telegraph orders largely obsolete by the 1970s. Where traffic density warranted it, multiple tracks could be provided, each with a timetable-defined flow of traffic which would eliminate the need for frequent single track-style 'meets.' Trains running counter to this flow of traffic would still require train orders, but other trains would not. This system was further automated by the use of Automatic Block Signaling and interlocking towers which allowed for efficient and failsafe setting of conflicting routes at junctions and that kept trains following one another safely separated. However, any track that supported trains running bi-directionally, even under ABS protection, would require further protection to avoid the situation of two trains approaching each other on the same section of track. Such a scenario not only represents a safety hazard, but also would require one train to reverse direction to the nearest passing point. Before the advent of CTC there were a number of solutions to this problem that did not require the construction of multiple single direction tracks. Many western railroads used an automatic system called absolute permissive block (APB), where trains entering a stretch of single track would cause all of the opposing signals between there and the next passing point to 'tumble down' to a Stop position thus preventing opposing trains from entering. In areas of higher traffic density, sometimes bi-directional operation would be established between manned interlocking towers. Each section of bi-directional track would have a traffic control lever associated with it to establish the direction of traffic on that track. Often, both towers would need to set their traffic levers in the same way before a direction of travel could be established. Block signals in the direction of travel would display according to track conditions and signals against the flow of traffic would always be set to their most restrictive aspect. Furthermore, no train could be routed into a section of track against its flow of traffic and the traffic levers would not be able to be changed until the track section was clear of trains. Both APB and manual traffic control would still require train orders in certain situations, and both required trade-offs between human operators and granularity of routing control. The ultimate solution of the costly and imprecise train order system was developed by the General Railway Signal company as their trademarked 'Centralized Traffic Control' technology. Its first installation in 1927 was on a 40-mile stretch of the New York Central Railroad between Stanley and Berwick, Ohio, with the CTC control machine located at Fostoria, Ohio. CTC was designed to enable the train dispatcher to control train movements directly, bypassing local operators and eliminating written train orders. Instead, the train dispatcher could directly see the trains' locations and efficiently control the train's movements by displaying signals and controlling switches. It was also designed to enhance safety by reporting any track occupancy (see track circuit) to a human operator and automatically preventing trains from entering a track against the established flow of traffic. What made CTC machines different from standard interlocking machines and ABS was that the vital interlocking hardware was located at the remote location and the CTC machine only displayed track state and sent commands to the remote locations. A command to display a signal would require the remote interlocking to set the flow of traffic and check for a clear route through the interlocking. If a command could not be carried out due to the interlocking logic, the display would not change on the CTC machine. This system provided the same degree flexibility that the manual traffic control has before it, but without the cost and complexity associated with providing a manned operator at the end of every route segment. This was especially true for lightly used lines that could never hope to justify so much overhead. Initially the communication was accomplished by dedicated wires or wire pairs, but later this was supplanted by pulse code systems utilizing a single common communications link and relay-based telecommunications technology similar to that used in crossbar switches. Also, instead of only displaying information about trains approaching and passing through interlockings, the CTC machine displayed the status of every block between interlockings, where previously such sections had been considered 'dark territory' (i.e., of unknown status) as far as the dispatcher was concerned. The CTC system would allow the flow of traffic to be set over many sections of track by a single person at a single location as well as control of switches and signals at interlockings, which also came to be referred to as control points.