Nor is it a given that the U.S., or even U.S. cities, can quickly attain a rail transit (or auto-alternative) percentage equal to the likes of New York.
But flip that picture around: It's borderline preposterous for anti-rail partisans to hold secure to the notion that "auto uber alles" can continue unabated; assuming continued growing demand for energy (conventional or alternative), coupled with other energy needs (heat or cool one's home? Desalinate one's water supply?), can auto-dependence, even aided by electricity or batteries or other fuel options, really resume a growth trend only recently upset after a nearly uninterrupted 90-year ride ever upward?
Freight railroads, particularly the Class I's, have offered rosy scenarios of the money to be made from even modest modal share capture from over-the-road options. Passenger railroads don't make money, but they certainly could lose less with more potential customers and/or customer reach, be it Amtrak, regional ("commuter") railroads, or urban transit networks—even before the externalities favorable to rail (land use, economic development) get factored in.
But is there any optimal threshold? If so, what might that be? How might it vary from place to place, or even by time of day or year? Portland, Ore., shouldn't automatically be copied by Portland, Maine, obviously. Other useful non-auto modes, like the bicycle, will see their percentages rise as well (in the case of the bicycle, the resurgence has already begun).
Realists—or merely those more short-sighted, as I am – might say the question is premature, given U.S. passenger rail's struggle to reassert itself as a serious modal option. But it's no longer 1974, and as rail (freight and passenger) resumes a more visible role in the U.S. transport mix, the idea of what's ideal will, eventually, merit consideration.