Signal box

Signal box

A signal box or signal cabin is a building from which railway signals and points are controlled. The term signal cabin is used in Ireland, parts of Scotland and in Australia while in North America, the term interlocking tower predominates.


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 or her control.

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 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 signalling system. Track circuits transmit train locations to distant control centres and data links allow direct manipulation of the points and signals.

While there are still hundreds of signal boxes in service all over Europe, the number in North America has shrunk to under a hundred. While some signal boxes may remain active in various niches, the future brings ever increasing levels of automation.


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 North America, abridged names have been assigned to some interlockings for brevity in verbal communications, e.g. 'Perryville' was shortened to 'Perry'.

On many railways it is common practice for signal boxes to be identified by a short alphanumeric code as well as by name. The name and/or code is usually prominently displayed on the exterior of the signal box, where it can be easily viewed by traincrews.

Control apparatus

Lever frame

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.

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.

Control panel

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:

;Individual Function Switch (IFS):A separate button/switch is provided for each signal and for each set of points. This type of panel is operated in a similar manner to a lever frame. The signalman must move each set of points to the desired position before operating the switch or button of the signal reading over them.

:This type of panel needs the least complex circuitry but is not suited to controlling large or busy areas.

;One Control Switch (OCS):A separate switch/button is provided for every signalled route. There will be as many switches/buttons per signal as there are routes (i.e. signalled destinations) from that signal. To set the desired route, the relevant switch or button is operated. All points within the route are automatically set to the required position.

:Individual points switches are provided, but they are normally left in the central position, which allows the points to be automatically set by the action of setting a route.

;Entrance-Exit (NX):This type of panel has one switch/button provided for every signalled route (except that some panels have separate 'entrance' and 'exit' devices). To set a route, the signalman operates the device for the 'entrance' signal, followed by the device for the 'exit' (destination) signal. All points within the route are automatically set to the required position and, provided all the points are detected by the interlocking in the correct position the signal will clear.

:Individual points switches are provided, but they are normally left in the central position, which allows the points to be automatically set to the normal or reverse position by the action of setting a route.

Similar principles of operation as described above are applicable throughout the world.

Visual Display Unit

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.

Present day

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 common away from the busiest lines.

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.


* Kichenside, G. and Williams, A., (1998), "Two Centuries of Railway Signalling", Oxford Publishing Co., ISBN 0-86093-541-8
* Vanns, M.A., (1995), "Signalling in the Age of Steam", Ian Allan, ISBN 0-71102-350-6
* John Armstrong, "All About Signals." [ Trains Magazine] , July 1957.

See also

*Area Signalling Centre

External links

* [ The Signal Box]
* [ Over 100 photographs of traditional mechanical signal boxes on Britain's rail network]
* [ A large photograph collection of operational and non-operational boxes remaining in the United Kingdom]
* [ List of active and recently closed North American interlocking towers]

Photo gallery

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Look at other dictionaries:

  • signal box — signal boxes N COUNT A signal box is a small building near a railway, which contains the switches used to control the signals …   English dictionary

  • signal box — n BrE a small building near a railway from which the signals and tracks are controlled …   Dictionary of contemporary English

  • signal box — signal ,box noun count BRITISH a small building next to a railroad from which the signals and tracks are controlled …   Usage of the words and phrases in modern English

  • signal box — ► NOUN Brit. ▪ a building beside a railway track from which signals, points, and other equipment are controlled …   English terms dictionary

  • signal box — noun a building from which signals are sent to control the movements of railway trains • Syn: ↑signal tower • Usage Domain: ↑Britain (for: ↑signal tower) • Hypernyms: ↑building, ↑ …   Useful english dictionary

  • signal box — UK / US noun [countable] Word forms signal box : singular signal box plural signal boxes British a small building next to a railway from which the signals and tracks are controlled …   English dictionary

  • signal box — Brit. a railway signal tower. [1820 30] * * * …   Universalium

  • signal box — noun (C) BrE a small building near a railway from which the signals and tracks are controlled; signal tower AmE …   Longman dictionary of contemporary English

  • signal box — /ˈsɪgnəl bɒks/ (say signuhl boks) noun a raised cabin above a railway line, from which railway signals, points, etc., are worked. Also, signal cabin …   Australian English dictionary

  • signal box — noun A building, typically adjacent to or spanning a railway line, from where signals, points and (sometimes) level crossings are controlled. Syn: cabin …   Wiktionary

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