Start in the early 1900s with severe taxation on automobiles, reinforce it during World Wars I and II with strict gasoline rationing and higher fuel taxes, do not build the Interstate Highway System and instead concentrate improvements on the rail system. That's a start.
Yup. You hit the nail on the head.
A good starting point is to make affordable housing and infrastructural upgrades into primary initiatives of the New Deal.
This could provide the means to rebuild much of the urban north's deterorating housing stock, as well as laying down the ground work to insure that the emerging suburbs will be zoned in a denser more mass transit friendly manner. While suburbanization is more or less inevitable traditional zoning restrictions will do much to halt the endless sprawl witness by OTL.
As for infrastructure, inter-urban rail could always stand for some subsidized upgrades. Likewise, federal funds could finally allow cities such as Detroit, Los Angles, Baltimore, or Cleavland to get long desired subway systems. Said infrastructure would provide a major incentive for denser post-war growth and likely further blunt suburbanization and white flight.