Signal Boxes and Interlockings

The earliest parts of the London Underground opened in 1863. From the beginning, signalling at each station (and sometimes between stations) was controlled by signalmen operating from boxes or cabins where the levers controlling signals and points were concentrated. All levers were interlocked to reduce the risk of conflicting routes or inappropriate signal indications being given. All the connections with trackside equipment were purely mechanical.

From 1905, the new electric tubes and newly-electrified steam lines used miniature lever frames at junction and reversing stations, and points and signalling were controlled by electricity and compressed air actuated by miniature levers in nearby signal cabins; the levers were still mechanically interlocked. At all other stations signalling was operated purely automatically. The outcome was a drastic reduction in the numbers of signal boxes required, beginning a lengthy trend.

From the 1920s, the technology allowed ever larger geographical areas to be controlled from signal cabins and larger cabins slowly replaced a multiplicity of smaller ones. After the Second World War many cabins (old and new) were converted to remote control operation from another location, and some were built like this from new. Further technical progress meant the physical interlockings were separated from the signalman who actuated equipment through push-button technology, often from some distance away. Many of these unmanned cabins (better described as interlockings) were built like this from new. More recently signalmen were superseded altogether, initially by automatic 'programme machines' and more recently by local computers. Where this happened overall control and supervision was undertaken by a central control room. The outcome has been the near extinction of the signal cabin and technology has moved on so much that traditional mechanical interlockings are disappearing rapidly and will all be gone within a decade.

This web site lists all signal boxes, other interlockings and other examples of controlled signals on the Underground have been listed in alphabetical order by location. Over 700 locations have so far been recorded by the work should in no way be regarded as in any way complete. Notification of errors and omissions, and any amplifications, are always gratefully received. One day, when its author is happier about its completeness and accuracy, it is the intention to make this more readily available to the public.

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