One of the earliest forms of fixed railway signal is the semaphore, the position of its arm and the colour of its lamp indicating the status of the line ahead.
I built this rough representation of a semaphore signal for the ‘Railways’ Secretary’s Challenge at the North East London Meccano Club. It has some errors, but it does include a working white LED lamp powered by batteries.
The rules of the challenge stipulated that models must be no larger than 4½” in any dimension.
Semaphore signal