The eastern corridor is the big exception. The east coast is congested, period. The interstate highways are clogged and will only get worse. There is nowhere to build new ones. Amtrak needs to get its costs down somehow; if it weren't so danged expensive, there would be a much bigger market.
The Amtrak trains in the northeast are all booked solid every day. I just booked a train from Boston to D.C. for $158 round trip, so it is not a cost problem. The present system cannot add anymore intercity trains, not enough room on the tracks. Most of the choke points are bridges and tunnels.