Gas tax doesn't even begin to cover the externalities of burning that gas. Vehicle taxes go entirely towards the administrative costs of vehicles (i.e. DMV) and also don't cover externalities. Sure the road is already built but it was funded by all tax payers, not all of which drive cars. And roads require maintenance, also tax payer funded. Society at large subsidizes cars and drivers, giving them huge chunks of land while making most cities inhospitable to pedestrians, causing almost as many deaths as guns in the US. A giant, heavy, extremely dangerous machine is just about the worst way to transport people. Maybe we should stop subsidizing it entirely and taxing it to lower use like we did with cigarettes?

> As far as cost-of-ownership, the trains are actually the thing being subsidized here, not cars.

Exactly. In fact, if you try to find a single example of passenger rail that is financially sustainable you will likely fail. The only reason passenger rail exists is because it is massively subsidised. It is inherently flawed as a mode of transport.