Well, they're not always twiddling their thumbs. Every time the train comes in to the station, they have to open the window and point up at the zebra-striped sign.
That's not literally twiddling your thumbs, but I'll leave it as an exercise to the reader to determine whether that serves any purpose on a fully-automated train that couldn't be eliminated with no loss of functionality, safety, or convenience.
Most of the times?
What would happen if operators were twiddling their thumbs (or absent) the rest of the times?
Uber's self-driving car also has an operator "twiddling their thumbs most of the times", but I wouldn't want to eliminate the operator.