Once you get the actual human, 1000Hz tone at max volume. Or play a "numbers station" just to confuse the shit out of them...
It's not necessarily stupid that there's no authentication. This fails safe (train stops), not deadly
Trains already have a dead-man switch, generally in the form of a Big Red Button that has to be pressed within a certain time after a buzzer sounds (called an alerter).
The way that train brakes are applied is interesting - they respond to a DROP of air pressure in the brake pipe that goes from wagon to wagon. This is a fail-safe to force the brakes to apply if the line develops a leak. But what if the line has a clog or closed valve somewhere in the train? The dead-man switch in the locomotive would only cause the brakes IN FRONT OF the clog to apply - the radio system works from the rear of the train, so will apply the brakes BEHIND the clog. In an extreme situation, both the dead-man switch and the radio system can be useful.
The moon is a planet just like the Earth, only it is even deader.