Wouldn't the most sensible one be a high speed version of  the northeast corridor from Boston to DC, to be extended north to Portland and south through Richmond down to the NC Research Triangle and Atlanta? There is the problem of dealing with construction in a very densely populated area (much more so than France. However, it would make a lot of sense, especially with a twin set of stops, one every decent sized center - i.e. stopping in places like New Haven and Baltimore, the other just the major ones - Boston, NYC, Philly, DC, etc. Nobody would use it for the full route, but I'd imagine that with the right ticket price there would be enormous demand for shorter legs. And again, we're talking about an area with an enormous population.