The London docklands railway have a staff member on board who opens/closes the doors and starts the train moving. It's a less skilled (and therefore almost certainly cheaper) job than driving a train, but it's still a person onboard.

Apparently not all the trains in Nuremberg are staffed, so perhaps it does work? I suppose the train driver is often segregated from the passengers in a way which makes them almost absent anyway.

It will be interesting to see if completely unstaffed buses are adopted anywhere, and if that proves succesful. I'd imagine that security wise a staff member isn't really necessary during busy day time routes (but then conversely that's probably when they'd be the most cost effective and provide the highest utility).