I have tested the HSM on M4. It does not work; You can drive a HSM which starts at the Airport as I made and ATB island of the departing platforms, and you can also drive the HSM towards the Airport.
But then you start getting problems; at least on my computer, the HSM does not which to ZUB protection after you reverse direction. So when you arrive at the airport and you want to go back, the ZUB protection won't kick in anymore.
Therefore, I would advise to use the HSM on ATB-only tracks, and not on M4 (or M5). And ofcourse, it is usable on the railways.