The other passengers serve as the social discouragement in that instance. Most human-driven rail transit isn't monitored by rail employees.

I imagine truly autonomous rides without a specialist on board will have plenty of cabin cameras with clear view to identify the passengers. With consequences in mind, does that work in a similar way to the social discouragement of other passengers?

What about elevators? Sure it's sometimes shared, but you can also ride alone.

I'm not saying I agree it will be a big problem - but graffiti, littering, urination definitely happen in public lifts.

I imagine though that it'll just be something you can report when it comes along, then it gets sent off for cleaning, they pull up its recent rides, and send you a different car.