> given that it's so heavily subsidized by the government
It's not. Until the recent infrastructure bill, Amtrak didn't even have a dedicated yearly budget. Every year they would beg Congress for some money, get some amount less than that, then prioritize what they could given the funding. Moreover roads in the US never pay for themselves, they're just written off as an "economic driver" rather than expected to pay for themselves. Very few roads in the US are toll roads and much of the US is vehemently against toll roads. Fundamentally, public infrastructure is public infrastructure. The public pays for public infrastructure. If we expect Amtrak to be able to fund some non-trivial percentage of its infrastructure costs then we're placing an expectation on Amtrak that the US road and highway system does not have.
Amtrak hasn't built much new rail. Amtrak maintains its rail and rolling stock and buys new rolling stock. Most of Amtrak's rail routes were originally made by the private rail companies that owned American rail during the Industrial era. Even the lines leased by freight rail was originally made by private rail companies. These companies were going bankrupt and the US government created Amtrak as a holding company to deal with selling off or winding down existing rail assets.
It's not. Until the recent infrastructure bill, Amtrak didn't even have a dedicated yearly budget. Every year they would beg Congress for some money, get some amount less than that, then prioritize what they could given the funding. Moreover roads in the US never pay for themselves, they're just written off as an "economic driver" rather than expected to pay for themselves. Very few roads in the US are toll roads and much of the US is vehemently against toll roads. Fundamentally, public infrastructure is public infrastructure. The public pays for public infrastructure. If we expect Amtrak to be able to fund some non-trivial percentage of its infrastructure costs then we're placing an expectation on Amtrak that the US road and highway system does not have.