The "Texas triangle" is one example of some good city pairs, which are a lot more practically situated than the pie-in-the-sky NYC-LA route people always bring up as a reason HSR won't work in the U.S. due to distances (and this is not the only such set of routes, just a particularly high-volume set).

Houston-Dallas is a very high-volume city pair forming one leg of the triangle, of length around 250 miles. People do fly on this route, but most people don't, because between the airports' relatively inconvenient location (especially Houston's), security, delays, etc., it doesn't save you that much time. So most people just drive. But driving is just far enough, and stress/traffic-filled enough, that many people who drive that route regularly hate doing so (it takes 4-6 hours, depending on how good a day you're having, and much of that is with wall-to-wall traffic containing a heavy volume of trailer trucks in the mix). I've done it many times myself, and I'd love to take a train instead. For me personally, it wouldn't even have to be that fast: if the train could get me there reliably in 3 hours (i.e. average speed 85 mph, not even talking about Shinkansen speeds), I'd be very satisfied. Related things can be said about the other two legs of the triangle: San Antonio - Houston (200 miles) and the San Antonio - Austin - Dallas corridor (80+195 = 275 miles).

The topography here is also more friendly to rail than, for example, LA-SF, being mostly flat and open and with shorter distances. But unfortunately the state-level political geography is less friendly (at a local level rail is popular in all four of these cities, but the state is another matter).