A signal box or signal cabin (interlocking tower in US) is a building from which railway signals and points may be controlled. Originally all signalling was done by mechanical means; cables or rods, connected at one end to the signals and points and at the other to the signal box or other triggers, run alongside the railway. This required signal boxes to have complex arrangements of cabling, levers and pulleys. The guts of this system is the signalling frame, wherein one finds this complex cabling arrangement and linkage to levers and controls.

Nowadays, some old-style signal cabins can still be found. Some still control mechanical points and signals. More often than not, the groundframe has now been removed or is out of use. This is due to widespread electrical signalling. Most modern countries have few, if any, mechanical signalling remaining in the rail system.

The modern control centre has replaced widespread signal cabins. These centres, usually located near main railway stations, control the over track network using computer systems. One such system is CTC, Centralised Traffic Control. 'Circuits ' exist, electrically linked sections of track; these replace the old style 'blocks ' of track. The CTC system uses these to determine the location of trains on the system. When a train is on a section of the network, it completes the local 'circuit' and the nature of the electrical signals will show whereabouts exactly the train lies.

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• North American railroad interlocking towers and cabins