This is alot harder to do politically than it sounds like in principal. Because mass transit is most effective in large cities, any federal effort to subsidize it would look like a giveaway to coastal elites, and state efforts would run into a local version of the same problem. To add to that, political conservatives are ideologically opposed to mass transit (see this George F. Will
column for a slightly hyperbolic rundown of the arguments against it), and there isn't a natural constituency for expanded transit in the same way there is for budget priorities like schools and farm subsidies, because the people who would benefit don't see themselves as beneficiaries.
If you want federal investments that are large enough to make a difference, you probably do need to make driving less attractive. The best ways to do it from an AH standpoint are to extend either the 1970s energy crisis or the high gas prices of the mid-late 2000s.