This is a circuit example of the relay logic for a railroad wayside signal. In this example are four switches that act as track blocks as activated by the dispatcher. There are also six LEDs acting as the 2 3-color displays which the train crew would observe.
The possible aspects for this signal are: Stop, (red/red) Approach, (yellow/red) Approach Medium, (yellow/green) Advance Approach, (yellow/yellow) and Clear. (green/red) This type of signalling is used for Speed control signalling, typically in the Eastern US.
