I forgot to mention a couple of things. 1st. Which ever end was receiving regular code pulses, would send a very short "inverse code pulse" back, as an echo. This echo allowed both ends (not just the current RX end) to determine occupancy of the entire stretch between headblocks. Note also that if any opposing signal was cleared or approach locked anywhere along the way, this echo would be blocked or suppressed, locking out traffic opposing that signal.

When mag-stick relays were used to set traffic direction, it was usually actuated by command from the dispatch office panel. So long as the traffic section (headblock to headblock) was clear of any occupancy and no opposing signals were cleared or approach locked, the commands from the dispatch office were allowed to reach the mag-stick and change its state. Otherwise, these commands were blocked, and the mag-stick stayed as it was.

One exception (in many installations) to this was, that circuits were arranged so arranged that after a train had entered the section and cleared the OS, the mag-stick at the entering headblock could be reset for traffic in the other direction, even while the traffic section was still occupied. But then it could not be set to enter the section again until the entire section was clear.

This feature was very useful, for reversing a train back the way it came at any point along the way. But was much more commonly used by the dispatcher / CTC operator, to make ready to align an apposite direction route, for an opposing train after a meet at the next siding up the line. It caused no interference or risk to the departed train, but was convenient for the CTC operator.

But it would allow the headblock facing-point signal to clear behind the train. In many installations (but not all) it would also permit intermediate signals to clear behind the train as well. But if the dispatcher did not reset the mag-stick, no signals could clear behind a train. So the issue of when that would happen and when it wouldn't, was quite dynamic, even when circuits were so arranged to allow it. And they were not always so arranged either, in which case, it never happened.