The main signal informs the train driver whether they must stop or can proceed. Some main signals also act as distant signals for the following main signal.

A main light signal consists of two to five light apertures and can display seven different signal aspects. It is the most common type of signal and is found both at stations and between them. In addition to the basic function of allowing or prohibiting passage, the main light signal also indicates the permitted speed and, if applicable, what the next signal shows (built-in distant signaling).