It's more than this. It's a federally owned and operated passenger railroad. No other federal agency is expected to pay its own way. Why should Amtrak?
"Motorists pay the full cost of federal highway funding."
Not true. Not even close. Locomotives use diesel fuel. Billions of gallons of it. A few years back, I read that only the US Navy used more diesel fuel than Burlington Northern. These billions of gallons each have a tax on it. Tax $$ that goes to federal highway funding. Fedreal highways used by trucks. What other industry is forced to subsidize it's competition?