b) In a point based train control system, communication only happens at specific points. That's why it is called this way. On it's own it is safe, because in order to do anything non safe, like passing a red signal, you need to pass such a point. In order to violate a speed limit, you would need to pass the start of the speed limit, which also is such a point.
If you operate a line based control system, you are constantly making collisions from the view of the point based control system. The moment the line based system breaks down, the point based control system takes over and the only safe choice to do in the case of a collision is to immediately stop all trains. That's exactly what it did.
b) In a point based train control system, communication only happens at specific points. That's why it is called this way. On it's own it is safe, because in order to do anything non safe, like passing a red signal, you need to pass such a point. In order to violate a speed limit, you would need to pass the start of the speed limit, which also is such a point.
If you operate a line based control system, you are constantly making collisions from the view of the point based control system. The moment the line based system breaks down, the point based control system takes over and the only safe choice to do in the case of a collision is to immediately stop all trains. That's exactly what it did.