[jacquesm]

Trainstations are somewhat controlled, bus-stops are not, they can be just about anywhere.

[masklinn]

Which is relevant to the difficulty of designing a self-driving bus, but says nothing about any driver responsibilities outside of driving.

[robryk]

Not really. In case of a problem on board a train (unrelated to train's motion; for example, a medical emergency or a rowdy passenger), the train can usually be met by someone at the next station. In case of a bus, much more often there is no one to meet it at the next bus stop.