The technology exists to have only trains get close enough to everywhere that you can only use train and feet? Honest question, can you get by with only trains even in Japan?
I get by in an EU capital city just fine using only trains.
An EU capital city, sure, but having ONLY trains implies being able to go to any city and any suburb and any neighborhood by train.
I think the hybrid approach here might work well. Allow self-driving cars, but limit their speed to one in which they are very clearly safe, maybe something like 10 to 20 mph currently. This would cause most people most of the time to use mass transit, and if the last mile(or first, getting to the train station) of their trip required it they could opt for it.