If it can go 500 mph and has a solid purpose. Why not???
Because it costs as much as 40 airplanes that can go 500mph? And will take 20 years to build?
Or because you need transportation SYSTEMS -- not a feelgood souvenier attraction that will 40 years to show "a profit"?
But mainly because NO ONE will choose an airline ticket with more than one stop for a reason.. And a high speed train with limited access needs a dependable clientele that will REGUARLY buy tickets. There's not too many of those corridors in a country this big. And the ones that EXIST and are CANDIDATES will never support 500mph except in very limited parts of it's path. Like the SF to LA run for instance. Or NY to Wash.