2. Main signage functions

Historically, as soon as several trains ran on a given line, they had to be able to overtake or pass each other (a role played by switchgear command and control), and they had to know where they were in order to avoid accidents (overtaking, nose-to-nose, slinging, etc.), which meant knowing the position of these trains with a certain degree of precision (a role played by train detection systems).

Lateral signalling soon appeared along the tracks. More or less recently, this has been supplemented by signalling in the train driver's cab.

Train spacing devices and interlockings complete the panoply of basic railway signalling functions we're about to discover.