As I see it, the basic problem is that without cheap cars & fuel you simply can't put together a transit system that lets people go conveniently from one random point to another 20 miles away, on a whim and several times a day.

What might be possible is to rearrange things so that the average trip is much shorter. Think in terms of the average commute or errand being, say, three miles (one way). This is about the size of a small town. It would be within the range of an electric city car for those who require it, but it would also be easily bikeable. Walkable for many people, too.

This would suggest that there should be local employment and retail centers spaced five miles apart more or less. It would make sense for public transit to concentrate on connecting those local commercial centers to each other. People could still use their private vehicles or walk within their local areas.

I imagine this is the way things will have to develop. It doesn't discard our current infrastructure and doesn't require anyone to change their way of life unless they choose to do so. What it does require is reworking things so that each suburb and small town has a commercial center and local schools.