It may be useful to explain something about trains' brakes here.

The braking system of a train is, as a default, on, or braked. When the engine is connected, running, (and other conditions, such as switches are right), air pressure is put into the system to release the brakes from their locked position. When the brakes are to be rapidly applied, the pressure is DUMPED, or for gradual braking, pressure is DECREASED.

The dead-man switch, of whatever nature, will release the pressure in the brake system in a hurry.

If a rear section of a train somehow were to break loose, it would break the air hoses, lose pressure, and that loose part of the train would brake automatically and be stopped. The main part of the train, still attached to the locomotive providing the pressure, can still roll because it still has pressure, although I expect it would be losing pressure from the now-broken hose, and thus because of the lowered pressure in the system the main part of the train would tend to be slowed.

Now, for a train to self-start from a station the locomotive would have to be running, and whatever switch or valve is involved would have to be turned on to allow air to the brake system. Even if someone were careless enough to leave the brake system charged, the dead-man switch ought to release the pressure after whatever little period of delay is built into it, and lock the train in place.

Dave Oesterreich