The math doesn’t really work out for your argument. For the DC metro, for example, the cost is about $1 per passenger mile.
https://www.transit.dot.gov/sites/fta.dot.gov/files/docs/300.... Fare revenue brings that down to a $0.53 cent per mile subsidy. One calculation of driving subsidies (relying on various studies that estimate the cost of parking, pollution, etc.) brings the driving subsidy to about $0.43 per passenger mile:
https://ggwash.org/view/10891/funding-amtrak-is-more-cost-ef.... (That’s from an urbanist, pro-transit website.) Electric cars cut that by more than half. (The direct subsidy for driving is just $0.02 cents per passenger mile.)
So if you didn’t have far revenue, each passenger mile of subway would receive about 4x the subsidy of a passenger mile of electric car travel. As it is, the subsidy is slightly higher for the subway than a gasoline car.