Nothing. Viennas subway has semi autonomous trains since 1960 or so. The driver just presses a “leave station” button after verifying nobody is stuck in the doors.
According to https://en.wikipedia.org/wiki/Vienna_U-Bahn#Fifth_expansion_... it seems that vienna isn't driverless yet.
You can quite often see the trains operate fully automatic though when the drivers are changing shifts at the end of the line. The driver typically leaves and locks the train, lets it reverse itself and then the next driver continues: https://www.youtube.com/watch?v=0eyjqS9lvNA