Centralized traffic control
These offices are usually located near the busiest yards or stations, and their operational qualities can be compared to air traffic towers.
These two mechanisms for control would be formalized by American 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.
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.
The development of Direct Traffic Control via radio or telephone between dispatchers and train crews made telegraph orders largely obsolete by the 1970s.
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.
Such a scenario not only represents a safety hazard, but also would require one train to reverse direction to the nearest passing point.
[citation needed] 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.
[citation needed] In areas of higher traffic density, sometimes bi-directional operation would be established between manned interlocking towers.
The ultimate solution to the costly and imprecise train order system was developed by the General Railway Signal company as their trademarked "Centralized Traffic Control" technology.
Although some railroads still rely on older, simpler electronic lighted displays and manual controls, in modern implementations, dispatchers rely on computerized systems similar to supervisory control and data acquisition (SCADA) systems to view the location of trains and the aspect, or display, of absolute signals.
Typically, these control machines will prevent the dispatcher from giving two trains conflicting authority without needing to first have the command fail at the remote interlocking.
6 miles (9.7 km) in length, it was installed by the Victorian Railways as a prototype for the North East standard project.
On other lines, CTC was installed between Upper Hutt and Featherston in 1955 and between St Leonards and Oamaru in stages from 1955 to 1959.
The older CTC installation from St Leonards to Oamaru was replaced in stages with Track Warrant Control in 1991 and 1992.
