Texas triangle would also be a viable region.
As my sibling comment points out, southeast US is marginally viable.
The Pacific Northwest (Portland-Vancouver) is also on the margin of viability.
The other parts of the northeast--Boston-Toronto and NYC-Montreal--are very definitely viable HSR routes. You might find some more strap-on routes to the main Northeast Corridor that I haven't included (Scranton, maybe?), simply because the NEC is such a phenomenally powerful generator of traffic.
Connecting the Midwest to the Northeast and the Southeast may also be viable, but again it's somewhat more marginal because the big city in the Midwest is on the wrong side of the region.