The united states only has a handful of 'large' cities. Most cities with larve populations are only populated because they encompass large geographies. A large city shouldn't be defined by population but by density and the united states has very few dense cities. Maybe only new York, Philadelphia, washington DC, San Francisco, Chicago, and Boston, and even some of those are a stretch.
But that's the ultimate problem to solve. Build dense cities, and people will (a) want to live there and (b) will want to use transit and (c) transit will make the most sense. The city density issue is one completely controlled by the government. Developers are incentivized to produce high-density housing, because it yields more money. Cities don't want it.
