Railway signal
A Signal is a mechanical or electrically operated device that indicates to train drivers when they must stop or may start.
Mechanically Operated Signals
Electrically Operated Signals
Tokens
Before signals were used, a driver was given a physical object called a token to authorise him to use a particular stretch of single track. Since there was only one token for each length of track, the driver could be confident that another train would not meet him from the other direction. The token system has now largely been replaced by signals.
