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On a rail transport system, signalling control is the process by which control is exercised over train movements by way of and block systems to ensure that trains operate safely, over the correct route and to the proper timetable. Signalling control was originally exercised via a decentralised network of control points that were known by a variety of names including signal box (International and British), interlocking tower (North America) and signal cabin (some railways e.g., GCR). Currently these decentralised systems are being consolidated into wide scale signalling centres or dispatch offices. Whatever the form, signalling control provides an interface between the human signal operator and the lineside signalling equipment. The technical apparatus used to control Railroad switch (points), signals and block systems is called interlocking.
With the practical development of electric power, the complexity of a signal box was no longer limited by the distance a mechanical lever could work a set of Railroad switch or a semaphore signal via a direct physical connection (or the space required by such connections). Power-operated switch points and signaling devices greatly expanded the territory that a single control point could operate from several hundred yards to several miles.Principles of Electric Locking by James Anderson As the technology of electric relay logic was developed, it no longer became necessary for signalmen to operate Lever frame with any sort of mechanical logic at all. With the jump to all electronic logic, physical presence was no longer needed and the individual control points could be consolidated to increase system efficiency.
Another advancement made possible by the replacement of mechanical control by all-electric systems was that the signalman's user interface could be enhanced to further improve productivity. The smaller size of electric toggles and push buttons put more functionality within reach of an individual signalman. Route-setting technology automated the setting of individual points and routes through busy junctions. Computerized video displays removed the physical interface altogether, replacing it with a point-and-click or touchscreen interface. Finally, the use of Automatic Route Setting removed the need for any human input at all as common train movements could be fully automated according to a schedule or other scripted logic.
Signal boxes also served as important communications hubs, connecting the disparate parts of a rail line and linking them together to allow the safe passage of trains. The first signaling systems were made possible by technology like the telegraph and block instrument that allowed adjacent signal boxes to communicate the status of a section of track. Later, the telephone put centralized dispatchers in contact with distant signal boxes, and radio even allowed direct communication with the trains themselves. The ultimate ability for data to be transmitted over long distances has proven the demise of most local control signal boxes. Signalmen next to the track are no longer needed to serve as the eyes and ears of the signaling system. transmit train locations to distant control centers and data links allow direct manipulation of the points and signals.
While some railway systems have more signal boxes than others, most future signaling projects will result in increasing amounts of centralized control relegating the lineside signal box to niche or heritage applications.
On systems where Morse code was in use it was common to assign control locations short identification codes to aid in efficient communication, although wherever signalling control locations are more numerous than mileposts, sequence numbers and codes are more likely to be employed. Entire rail systems or political areas may adopt a common naming convention. In Central Europe, for example, signalling control points were all issued regionally unique location codes based roughly on the point's location and function, while the American state of Texas sequentially numbered all interlockings for regulatory purposes.
As signaling control centers are consolidated it can become necessary to differentiate between older style boxes and newer train control centers, where signalmen may have different duties and responsibilities. Moreover, the name of the signaling center itself may not be employed operationally in preference to the name of individual signaling workstations. This is especially true when signaling centers control large amounts of territory spanning many diverse lines and geographical regions.
In most cases where the control locations are still in the field adjacent to railway tracks, the name or code of the control point is plainly labeled on the side of the signal box structure as an extra visual reminder to the train operators where they are. Moreover, wayside signals may also be equipped with identification plates that directly or indirectly indicate who controls that signal and that stretch of the line.
In many countries, levers are painted according to their function, e.g. red for stop signals and black for points, and are usually numbered, from left to right, for identification. In most cases, a diagram of the track and signaling layout is mounted above the lever frame, showing the relevant lever numbers adjacent to the signals and points.
Hand-powered interlockings were referred to as 'Armstrongs' and hand throws in the United States.
Power frames have miniature levers and control the signals and points electrically. In some cases, the interlocking was still done mechanically, but in others, electric lever locks were used.
In a few cases, signals and points were operated pneumatically upon operation of the appropriate lever or slide.
Similar principles of operation as described above are applicable throughout the world.
The modern control centre has largely replaced widespread signal cabins. These centres, usually located near main Train station, control the track network electrically or electronically.
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