It's a lot easier to pull a self-driving car out of service for cleaning than a six-carriage train. It's not unreasonable to expect that some combination of machine vision, Mechanical Turk and hands-on inspection will be able to detect a car with a soiled interior before it gets dispatched to another customer.

I imagine there will be plenty of teething problems and horror stories, but I don't think it's an insoluble problem.