Even simpler, there are systems that use a physical token. To proceed into a section of track, the train driver/guard must physically possess an object, typica

Even simpler, there are systems that use a physical token. To proceed into a section of track, the train driver/guard must physically possess an object, typically handed over at a station just before the section, and returned just after. https://en.wikipedia.org/wiki/Token_(railway_signalling) https://en.wikipedia.org/wiki/Token_(railway_signalling)