There is also the token-based control of bidirectional travel on single-track, in which physical posession of a token governs permission to enter a track block.

There is also the token-based control of bidirectional travel on single-track, in which physical posession of a token governs permission to enter a track block. Still used in various places. https://en.wikipedia.org/wiki/Token_(railway_signalling) https://en.wikipedia.org/wiki/Token_(railway_signalling)