The existing transporation infrastructure is almost entirely based on automobiles and airplanes. The railroad system in the Northeast carries a substantial fraction of the commuter traffice, and much of the railroad infrastructure built during the second world war is still in place so we could fairly easily implement a 1945-scale railroad transportation system. But even during the depression, most people had cars.

On the other hand, it would take no change to the roads to run busses. And electric busses powered by overhead wires are practical and not too expensive, so that would be a pretty easy step to take to cover transportation in the exurbs. Truly rural areas are more of a problem, but there aren't many people living in rural areas anyway.