On a rail transport system, signalling control is the process by which control is exercised over train movements by way of railway signals 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 (British), interlocking tower (North America), signal poste (France) and signal cabin. 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.
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Originally, all signalling was done by mechanical means. Points and signals were operated locally from individual levers or handles, requiring the signalman to walk between the various pieces of equipment to set them in the required position for each train that passed. Before long, it was realised that control should be concentrated into one building, which came to be known as a signal box. The signal box provided a dry, climate controlled space for the complex interlocking mechanics and also the signalman. The raised design of most signal boxes (which gave rise to the term "tower" in North America) also provided the signalman with a good view of the railway under his control. The first use of a signal box was by the London and Croydon Railway in 1844 to control the branch line to Bricklayers' Arms.[1]
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 points or a semaphore signal via a direct physical connection (or the space required by such connections). Power operated switch points and signalling decides greatly expanded the territory that a single control point could operate from several hundred yards to several miles.[2] As the technology of electric relay logic was developed, it no longer became necessary for signalmen to operate control devices 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. Computerised 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 signalling 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 centralised 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 signalling system. Track circuits transmit train locations to distant control centres and data links allow direct manipulation of the points and signals.
While some rail systems have more signal boxes than others, most future signalling projects will result in increasing amounts of centralised control relegating the lineside signal box to niche or heritage applications.
In any node-based control system, proper identification is critical to ensuring that messages are properly received by their intended recipients. As such, signalling control points are provided with names or identifiers that minimise the likelihood of confusion during communications.
In the United Kingdom and other Commonwealth countries, each signal box is assigned a unique name based on the geographical locality. Where multiple signal boxes existed at one locality, suffixes such as "West" and "East" were added to the geographical name, or they would be individually numbered, e.g. "Stockport No. 1", etc. In very remote areas, the name may be derived from the lineside mileage, e.g. "73rd Mile".
In Central Europe, it is common to issue signalling control points with a regionally unique location code based roughly on the point's location and function.[3] The signalling locations may have longer, full names associated with them, but the identification codes can be employed more efficiently.
In North America, signalling locations were most often provided with one or two digit telegraph call letters that were locally unique to the rail line or division. These letters could stand for the control point's location or function as a form of mnemonic aid, but sometimes they were simply randomly or problematically chosen. The purpose of the short IDs were to reduce overhead when transmitting messages in Morse code. As time went on and voice communications replaced the Morse code the codes began to be replaced with more human recognizable names, again often based on geographic location. As control passed from field operators to centralised dispatchers, the dispatching desks took on identifiers that could encompass geographic areas or entire routes as well as simple numbers or programmatic identification codes.
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 labelled 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.
The earliest signal boxes housed mechanical lever frames. The frame was usually mounted on a beam beneath the operating floor. Interlocking was attached to the levers, which ensured that signals showed the correct indication with regard to the points and were operated in the right order. Wires or rods, connected at one end to the signals and points and at the other to levers in the signal box, ran alongside the railway.
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 signalling layout is mounted above the lever frame, showing the relevant lever numbers adjacent to the signals and points.
Handpowered interlockings were referred to as armstrongs and handthrows 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.
In a signal box with a control panel, the levers are replaced by buttons or switches, usually appropriately positioned directly onto the track diagram. These buttons or switches are interfaced with an electrical or electronic interlocking. In the UK, control panels are of the following types:
Similar principles of operation as described above are applicable throughout the world.
Modern signal boxes nowadays tend to be provided with VDU based, or similar, control systems. These systems are less expensive to build and easier to alter than a traditional panel. In the UK, large modern signal boxes are typically of the Integrated Electronic Control Centre type. Variations of these control systems are used throughout the world.
While rare, some traditional signal boxes can still be found. Some still control mechanical points and signals, although in many cases, the lever frame has been removed or is out of use, and a control panel or VDU has been installed. Most modern countries have little, if any, mechanical signalling remaining on the rail system. Both in the UK and Ireland, however, mechanical signalling is still relatively common away from the busiest lines; there is also a considerable amount in the former East Germany. Traditional signal boxes can be found on many heritage railways.
The modern control centre has largely replaced widespread signal cabins. These centres, usually located near main railway stations, control the track network using electrical or electronic systems. One such system is CTC, Centralised Traffic Control.
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