US population can be approximated as relatively dense populations in California, the Northeast, a central portion of Texas, and parts of the Midwest, with rural

US population can be approximated as relatively dense populations in California, the Northeast, a central portion of Texas, and parts of the Midwest, with rural Europe-ish levels of density (with some greater pockets of density nestled within) in the rest of the country ~east of the Mississippi (and a lesser degree in the Pacific Northwest), and truly empty regions basically everywhere else. In terms of where there should be lots of trains that there isn't, it's largely the Midwest outside of Chicago, Texas Triangle, and the Southeast, all of which could probably support hourly intercity HSR trains if they were competently built (although especially in the Southeast, this is going to be restricted basically to a single corridor).