Centralized Traffic Control - Background

Background

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. Train orders would supersede the timetable if modifications needed to be carried out on the fly. However, due to the limitations of communication technology the only means for those able to make strategic routing decisions to communicate with the train crews was via telegraph. The telegraph required fixed infrastructure and skilled operators so the system of block operators relaying routing information in the form of train orders was established. Trains would operate according to timetable unless provided with train orders via a local operator which were available at intervals along the route.

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 Mexican standoff 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 double 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.