There are plenty of places in the US that high speed rail would make sense. Even in places you wouldn't expect it. A bunch of regional airports pretty much just shuttle to nearby hubs and would be way more convenient for people if it was just a train. Take CLT to GSP or GSO. SBA to LAS. High speed rail would make a lot of sense there even if they're low density suburbs like the first example.
At least in fantasy land where it doesn't cost $1T worth of graft to build anything in this godforsaken country 