Relay hut as deployed by NSWGR nearby automatic signals in the mid 20th century.
Originally released for co-siting with 'NSWGR 3-position semaphore, upper quadrant, automatic signal' <kuid:368725:65038>.
In the era before integrated circuits, electro-magnetic relays and other discrete components were combined to form electrical circuits that stored and processed information about detected states in the real world. In this case the relay circuits detected the state of track occupancy in the 2 track sections in advance of the automatic signal controlled from the hut. This information was transformed into electrical instructions that set the state of the signal. For a semaphore signal, these instructions controlled the motor that set the position of the semaphore.