Hamburg to Munich is 775km on the autobahn. Some places just aren't geographically feasible to build brand new, straight tracks. Plus the train stops at stations to load/unload passengers, and it also takes time to accelerate back up to speed. The newer ICE rollingstock is capable of doing 330km/h but most of the time it'd never go that fast. Fastest I've been in was about 260km/h I think. Deutsche Bahn limits their speed to 300km/h AFAIK.
775km is about a little less than 500 miles. So which major US cities less than 400 miles of distance (that already have good existing public transportation) can we connect for the most bang for the buck? How would we make sure that the systems operate without too much taxpayer subsidies? As gas prices go up (which they sure will in the not so distant future), maybe people will start looking at alternatives?
If you take into account that the next generation of HSR rolling stock will be able do do 350-400km/h you can cast a somewhat wider net if desired.
The systems that stand the best chance of being built already fit your distance criteria. California: San Deigo to San Francisco, Chicago Hub (out to Minneapolis, Madison, Milwaukee, St. Louis, Detroit, Indianapolis, Cleavland, Cincinnati), and the NEC from Washington to Boston.
These systems would also have the benefit of reducing the load on our airports by diverting short haul trips to the rail instead of hogging runway and gate capacity at already overstretched airports.