Around the world governments "nationalize" what they allow themselves to, and at each level they nationalize the most salient and notable industry that's not too small to be small potatoes. In the U.S. the federal government doesn't allow itself to nationalize anything, the States do allow themselves but they can't bring themselves to hurt their industry as they compete with other States, but cities don't see themselves as competing with other cities, and cities allow themselves (and the States allow them to) to nationalize public transportation.
Take Argentina where a strong national government has at times nationalized steel production, oil production, etc., but they wouldn't deign to bother with nationalizing bus service -- it's like it's beneath them -- and so Buenos Aires has one of the most fantastic privately operated public bus systems in the world. You never have to wait more than a few minutes for a bus during business hours. But in the U.S. you're lucky if buses run more often than every 30 minutes at rush hour.
Do you want Americans to not drive their cars so much? Fine, it's easy: allow private companies to operate all public bus services, and also to operate small buses without set routes (a sort of Uber of buses). If you insist on the cities running public transportation then you can be sure that the public transportation system will never ever be good enough that Americans will be happy to relinquish their cars.
It's that simple.
And no, trains won't cut it. Laying tracks down is unbelievably expensive, will never pay for itself, and you can't ever change them afterwards, and you won't be able to place them where people can use them because that would be way too disruptive unless you make it subways, and that's even more unbelievably expensive.