If a train driver falls asleep, what happens to the train?

There is something called Dead Man’s Switch which does the magic. In the realm of trains, it is called an “Alerter”

Various control logics are in place so that this does not malfunction. But, to put the logic in layman’s words -

The Operator has to make some or the other movement to prove that he is in control of the train.

In condition like the constant speed cruising, there would not be much handle movements. So, he has to simply press this “Alerter” switch every now and then when prompted.

If in case the alerter switch was not pressed because the operator was incapacitated for some reason (may be unconscious or he was forced out of cab), then immediately the locomotive is powered down and emergency brakes are applied. this brings the train to a standstill.

Yes, we really do not want a run-away train. 4500 hp or multiples of that power with 5000 tons of load or more - that is not an easy thing to stop from outside. anything that would come in its way would vaporize (figuratively).