Traffic is irrelevant for trains. It has some effect on buses, which can be mitigated with HOV lanes. (Usually the problem there is frequency and a lack of express service). At the distances most people need to travel to reach their workplaces from affordable housing, walking and cycling are never going to be pleasant.

Banning cars from a city is a straight-up transfer of quality of life from the suburban masses to the richest in the center.

Drastically increasing high-rise housing supply in the center is a much more meaningful step.