From Christian Wolmar’s blog: Why are roads favoured by the right and trains by socialists?
An interesting question, I posted a reply, repeated below.

“From the US, I think part of the problem is the definition of “subsidy”. Here, auto users pay a user fee, most of which is in the form of a gas (petrol) tax, that is dedicated (hypothecated) to road construction, and pays in most places essentially 100% of the cost for major roads (freeways, state highways). (Local roads are largely paid for with property tax, but you would have these even without cars). So rather than thinking about it as a public subsidy, it is a service in exchange for a fee. In contrast public transit users pay about 1/3 of the operating cost (and about 0/3 of the capital cost) in most systems, the remainder is paid for out of general funds, dedicated sales taxes, and from highway user fees. The system is thus more subsidized by non-users. Also in the US 90+% of taxpayers are regular auto users, about 1% to 2% are regular transit users, so the cross-subsidy from transit users to highway users when using general revenue is relatively small and the cross-subsidy from highway users to transit users is relatively large. All of which sets the stage for the left/right divide. Things that are subsidized by the general public for the disadvantaged few (and riders of buses generally have much lower incomes than average, trains are different) are consistent with a “left”/Democratic point-of-view. People left to their own devices paying for what they use is a more “right”/Republican point-of-view. Trains, especially commuter trains, have attracted Republican support. This is because the users are well-to-do suburbanites who often vote Republican. Transit advocates endorse this as a way to broaden the base for transit support (though of course it will take resources away from other transit investments).”