That's a half-true statement. There's demonstrated demand for good train service in the US. Amtrak ridership is bound by its skeletal fleet, route map, and frequencies, grounded in short-term politics that no other federally-financed interstate mode of transport constantly faces.

Short-haul air travel invariably contracts (along with the slower trains) as real high-speed rail opens and grows to be even faster and along longer stretches. Japan, South Korea, Taiwan, and Spain have all experienced this. Contrary to popular perceptions, much of the US is sufficiently ripe in population density and economic activity for HSR to supplement Amtrak and relieve certain air corridors. Every mode has its purpose, which is why Bob Crandall (ex-AA), David Barger (ex-B6), and David Cush (VX) have all come out in favor of HSR in the US.