We mostly don't subsidize roads. We publicly pay to build them and maintain them directly, and anyone - including transit authorities - can use them for pretty much any transportation purpose. Having transit users pay for all the capital and operating costs of their rides - equipment, fuel, maintenance, insurance, etc - would essentially be the same situation facing private auto users - buy a car, fuel it, maintain it, insure it, etc. We could add or not add road tolling for both classes of user. Rail routes would have to remain an exception covered by the transit users unless someone figures out a way to share the rail routes with private auto users.