When you say "Interstate highways" do you refer to highways that bisect states or merely highways that are labeled as "interstate?"

I-96 is completely self contained within Michigan.

Would interstate commerce also mean that rail should be funded through federal subsidies? Or does cargo by rail have to fend for itself while by road is supported by federal taxes?

If you really want to break it down, wouldn't completely eliminating federal funding of everything be the only true model for actual competition? If a locality community or state wants to compete, then they can levy an appropriate tax amount. Sure, communities such as Detroit may be at a bit of an advantage over those with fewer residents, but then so be it. Hell, if I want a 16 lane superhighway that goes from point A to point B, I should have every right in this world to take whatever money I can rightfully get, buy the land and build my road.

I support federal regulations on things & usage of taxes. I just think many of the things our taxes are used on help to make it more difficult to make a better society. [[funding roads at a much higher rate than public transit, for example)