A signalling system where trains are kept separated by the time on clocks.
Where timetables are used, trains should arrive and depart on a strict schedule that caters to all known movements. Where no timetables are used, trains are provided an amount of time to (sufficiently) clear the block before the next train could follow.
Success in this system depends on all clocks being accurately synchronised, trains not breaking down (or apart) and all trains travelling at a uniform (or sufficient) speed, and all train movements being known — none of which could be uniformly guaranteed.