That's not really true, is it? I rarely have any reason to personally drive the highways of Iowa (despite living in Illinois next door), but presumably benefit directly and in a variety of ways from their availability to trucks. Amtrak can make no such claim.
I don't know if it's 100% true that Amtrak benefits only the areas it's in. People who take the train instead of flying (less carbon emissions) or driving (less interstate maintenance, paid for out of the federal budget, less gasoline consumed) are providing benefits to everyone.
Movement of people has economic benefits just as movement of goods does. If anything, in a service-based economy like that of developed countries, movement of people is more important.