Tower or building housing equipment for operation of points and signals in a particular section of a route.

a building from which signals are sent to control the movements of railway trains

A building where signal and points operating-levers are centralised. On the LNWR signal boxes were more usually called "signal cabins". The largest signal box on the LNWR was Euston No.2 (opened 1891) with 288 mechanical levers, one of the largest in the country, but other important signal boxes might have over 100 mechanical levers and two or more signalmen to work them.