What are single-wire and double-wire signalling?
‘Single-wire’ apparatus, as the name implies, utilizes a single wire or cable connecting the signal lever at the cabin or elsewhere where the signal frame is located, to the actual semaphore mechanism on the signal post. Operating the signal lever to take the signal off causes the transmission wire to be pulled, moving the semaphore arm to the required aspect. To reverse this and change the signal aspect to a more restrictive one the signal lever is moved back, and the semaphore arm moves back because of gravity acting on the semaphore mechanism (in some cases there may be appropriate counter-weights for this). In single-wire transmission, a signal can be pulled for up to 900m. A gain stroke wheel may be inserted at the foot of the signal lever to increase the lever stroke, or a so-called ‘facile stroke lever’ may be provided. In these cases the distance over which the signal can be pulled may be as high as 1080m. In ‘double-wire’ transmission, the wire that operates the semaphore loop
