The Northeast Corridor, which the GP is talking about, is a bit of a special case. Amtrak owns the tracks from NY to DC (states own the CT/MA part) and they're good enough to go a bit faster than the otherwise-usual 79 mph speed limit. This part of Amtrak is, IIRC, fairly profitable. https://en.wikipedia.org/wiki/Northeast_Corridor

In the rest of the country, Amtrak is indeed stuck on freight lines, and service is often slow and unprofitable, but it's not entirely on them either. Passenger traffic is supposed to be prioritized, but the freight lines are not great about it and have been fined for not doing so (though, not enough to make them do it either). There are also weird political considerations about routes.