The US is a massive country and in most of it rail is a niche. It's cheaper and faster to go by plane even counting security, getting there, getting from there and so on. Worst of all you need a car as local public transit is lacking so arriving in the middle of downtown via train is actually a negative as you can't easily rent a car.

Amtrak has proper service in the areas where it's economically viable like the northeast. The rest of it loses money and is basically there because it's legally required.

edit: San Francisco to LA, a relatively short trip as far as the US goes, is the same as going from London to Scotland. Seattle to LA is the same as London to Rome. That's not counting cross-country which makes those distances look small. Rail just isn't competitive at those distances.