I think most of this demand is already covered by the popular Northeast Corridor. It should absolutely be upgraded, but outside of it, I think everything's either too small or too far to effectively compete with air travel.
There definitely are city pairs or maybe triplets here and there. North Carolina was mentioned somewhere upthread. I've actually taken Amtrak from Raleigh to Charlotte. Could just walk to the station from downtown Raleigh. Did need a cab at the other end to get to downtown Charlotte but it was closer than the airport.
I know people take the train between Seattle and Portland. I'm sure there are other examples.
There are four main regions that have viable HSR outside of the NEC. These are:
* The CA region (SF/Bay Area/Sacramento/LA/Las Vegas/Phoenix).
* Texas triangle
* France TGV-like lines in the Midwest, centered on Chicago, connecting to Minneapolis (via Milwaukee), St. Louis, Detroit, Cleveland, maybe Indianapolis, Columbus, Cincinnati
* New England/New York outside of the NEC--that is, Toronto-Boston and Montreal-New York with timed transfers at Albany.
I would probably add Vancouver, BC -> Seattle -> Portland to that list, but I think that's probably right. The thing is, if we actually added viable rail to all of those cities mentioned, it would be a huge swath of the country, without needing to build a 200mph train through Wyoming.
Right now there are ~40 flights a day that go from PDX-SEA or SEA-PDX. Those are a waste of airport capacity and carbon emission. Both cities have the transit network to easily get people to the airport from wherever the HSR ends up for people who are using those flights to make connections, and could promote growth in the aggregate area in a way that doesn't exist now.
