Fast trains need well maintained tracks, specially built for the speed, and the curves also have to take that in mind. The advent of cars and later planes have pretty much undermined rail in America in terms of people transport and many lines are only suitable for slow-moving cargo trains, some as low as 10mph.
We're talking track that will bounce up and down out of the ground a good 18 inches into the air. I've seen this often enough with an approaching train in some sections. That track couldn't take 60 mph trains, let along several hundred mph.
America, may as well be trackless for the most part for rail that is in any condition and design for high speed rail. Buses have pretty much put an end to trains as a serious passenger mover, too bad they will never reach speeds that would make it a serious distance mover for anybody but the person with time on their hands, a tourist or sightseer.