Most of our rail network, outside the northeast corridor, is already vastly underutilized. Roads get more funding because that's what people use, and that's what's most convenient for 95% of people (who don't live in the downtown core of a major city) to use.
But regardless, you can get an idea from the EU. EU rail subsidies amount to 73 billion euro annually. EU rail carried 465 billion passenger-kilometers in 2017, or 300 billion passenger miles. That's a subsidy of $0.24 euro per passenger mile, even worse than Amtrak.
Roads are just really cheap to build and maintain. A 6-lane interstate highway costs $11 million per mile in urban-ish areas: http://blog.midwestind.com/cost-of-building-road. It's going to cost $1.4 billion to add four lanes to the 35 miles of the BW Parkway (which goes through a dense urban area connecting two major cities). At $40 million per mile, that's 1/6 of what it cost to build the Metro Silver line through less dense areas in an existing freeway median. That's 1/9 the per-mile cost of the purple line, a light rail line that mostly travels at grade without its own right of way.
I know you probably realize this but I think it's worth pointing out that even though it's cheap for the government to maintain roads, the total cost of transporting people by car on those roads includes much more than just the roads. Of course, it's politically difficult for the government to ask people to spend less on their cars and give the money to it instead so it can more efficiently and cleanly transport them by rail.
But regardless, you can get an idea from the EU. EU rail subsidies amount to 73 billion euro annually. EU rail carried 465 billion passenger-kilometers in 2017, or 300 billion passenger miles. That's a subsidy of $0.24 euro per passenger mile, even worse than Amtrak.
Roads are just really cheap to build and maintain. A 6-lane interstate highway costs $11 million per mile in urban-ish areas: http://blog.midwestind.com/cost-of-building-road. It's going to cost $1.4 billion to add four lanes to the 35 miles of the BW Parkway (which goes through a dense urban area connecting two major cities). At $40 million per mile, that's 1/6 of what it cost to build the Metro Silver line through less dense areas in an existing freeway median. That's 1/9 the per-mile cost of the purple line, a light rail line that mostly travels at grade without its own right of way.