Railway semaphore signal is one of the earliest forms of fixed railway signals. This semaphore system involves signals that display their different indications to train drivers by changing the angle of inclination of a pivoted 'arm'.

The semaphore was the first railroad signal to use electricity. The original semaphores used a two-position arm/blade to alert train operators if they must stop and stay (horizontal position), or could stop and proceed with caution (vertical position).

The signal is normally placed on the left side of the track with the arm directed over the offside. The standard arm is red with a white vertical band, although some older signals were plain red. To allow the signal indication to be seen at night, the arm is fitted with two lenses, duplicating the indication displayed.

Signals comprise a vertical post (of wood, steel tube, steel lattice or occasionally reinforced concrete) on which is mounted one or more ‘stop’ or ‘distant’ signal arms each comprising a rectangular ‘blade’ (of wood or steel) mounted on a pivot so as to extend to the left of the signal post (viewed from an approaching train to which the signal applies) at a height to facilitate observation by the driver of an approaching train.