Earlier quoted context omitted.
The general principle in railway movement authority is that there's an easy fail safe: stop moving. If nothing moves, there can be no collision. If something goes wrong? Stop the train. You can then figure out what went wrong and address it with no risk. Things shouldn't be going wrong, speeds shouldn't be being exceeded, etc.
>If nothing moves, there can be no collision. If no train on the line moves, sure. But stoping just the one train won't improve safety, and was historically the cause of many accidents. I remember just the one in Shanghai in 201x. One train stopped, the following train hit it.
Rail segments (blocks) are supposed to be automatically locked off while a train is on that segment of track. No two trains can be on the same block at the same time. If a train stops, the signal guarding that block will remain red, ordering the next train to stop before it gets anywhere close.