IIRC, the northeast corridor is the only profitable portion of Amtrak, and I think one of the very few runs of track that Amtrak actually owns.. and as a result, the only area that Amtrak spends money on, for anything, including laying its own rail.

Everywhere else it operates leased time on freight company rail lines.. so they can only, realistically, go where freight trains go. as building rail would be too expensive, and too politically inconvenient since they'd probably have to resort to overpriced buyouts and imminent domain to get a lot of the land to lay new rail on.