In the US, highways get an order of magnitude more government subsidy than transit (not just Amtrak but also urban subway systems, etc). Even if you treat the revenue from the gas tax as a user fee rather than a subsidy, there's still a huge subsidy coming from other kinds of tax revenue.
Chart and link to more info here: http://www.streetsblog.org/2008/07/22/highway-funding-the-la...