Correcting myself, the Coast Starlight line does NOT have 90 mph that I know of. It does have some double track.
Correct. The CS travels primarily over UP (former SP) trackage. ATS was installed primarily by the ATSF (now BNSF), which is why the PS and SWC both feature 90mph running. AFAIK, the UP/SP never installed anything beyond normal ABS/CTC, since the route (along the coast and through mountains) did not allow for higher speeds, anyway. In fact, relatively little of the track allows 79mph, not to mention anything higher.
At this point, I would focus on the HSR corridors the FRA has already defined. Cross-country 110mph or even 150mph service is not something that would, at this point, be a very good ROI--even at knocking transcon travel down to two nights or a full day, that long of a time is still not competetive with a 4.5-hour transcon flight.
Even in high-speed-rail mecca Europe, trains do a much smaller percentage of cross-continental service. You can take high speed rail from London to Rome, but even on their developed network, it still takes the better part of, IIRC, a day and a half. With LCCs offering nonstop service on the same route for pennies, pence, or Eurocents, the market for longhaul train service isn't as large as people here seem to think it is.
Develop the HSR corridors and then incrementally expand them to the point where they begin to intersect. As much fun as it would be to see our country building out a transcontinental high-speed railroad, it would be at such a tremendous cost and without a corresponding huge benefit that it really is just a pipe dream by some overzealous railnuts here!