With 150 mph maximum, coast-to-coast service could average about 36 hours (with stops, restrictions, etc.), that is, leaving in the evening and arriving the morning after the next, a one-day-only trip. Even with air travel, isn't a day pretty much spent after the some 500mph "rocketing" by plane (and all the effects that could entail), considering the six-to-nine hours flight time and not mentioning delays?
As far as I can tell, 110 mph max. trackage would be the least expensive and most readily operable level of high-speed service with basically only signaling upgrades needed. The even greater speeds, especially 150-200 mph lines, would increasingly need the grade seperations and other fairly expensive upgrades mentioned by others. Grade separations, of course, would be very good for railways in general. Up to now, however, the politics have been against those kinds of upgrades being financed by taxes. If the majority can get that changed, great! However, the wisdom in offering up our railways to probably irretrievable, even more likely wastefully expensive and un-responsive political control is, to me, questionable. Railroading has been and is inherently so efficient a form of transportation that it, thankfully, can thrive even in the face of necessarily tax-subsidized, very expensive motor and air transport.
If speed just for the sake of speed is wanted, I don't think making general taxpayers pay for it is appropriate, especially non-riding ones unable to use the service for transportation, nor is making them pay for photo run-by trains! With many of AMTRAK's schedules-- not leaving right after work hours (about 6pm) for the majority of workers and arriving before work (about 6am) in the next large city about 300-500 miles away, at conventional speeds-- perhaps railfanning, photography, dinner or leisure excursions or very occasional vacation travel is all AMTRAK is for? That purpose is certainly not financially sustainable without massive tax subsidy--taxes from those who are not able to use the service for serious daily overnight business, commuting or even shopping or visiting travel. Nor does that kind of arguably frivilous purpose do anything towards the very real abilities of rail passenger service to drastically cut our fuel consumption, pollution, traffic grid-lock and increase the survivability of passengers even in the event of a major mishap.
So, if someone, or a few, can hype the majority into taxing all those who are unwilling or unable to avail themselves of the present conventional-speed service or, perhaps even the new high-speed service-- considering the massive bureaucracy that will be "needed" to oversee the whole high-speed rail project (a bureaucracy so expensive in itself that it requires most of the subsidy just to pay its high-salaried, credentialled bureaucrats)-- then we could yet only barely have twenty-four-hour coast-to-coast service (yeah, that's all 200 mph maximum speed would actually realistically give us, and it be a strain at that!).
Anything leaving one coast and arriving the other coast, even next day, at the same time of day as the departure, wouldn't generally serve most points at the most usable times (large cities every 300-500 miles or so apart every twelve hours, creating corridor-like yet nationally interconnecting through routes optimized for all purposes of rail travel). That would require a layover of one more night before departure since there being only morning departures for next-morning arrivals. Why not just leave conveniently in the evening and spend one day (at 150 mph) or two days (at 110 or, even 90 mph) between coasts. Maybe a new definition of "limited" service-- with limited stops perhaps only every 100 miles or so, which is what some of AMTRAK's current "limiteds" have in their schedules as inherited from the heritage railroad passenger services-- could be offered with the super high-speed trains. Still, I don't think anything greater than 150 mph service is especially useful. I think, on the whole, supporting the very simple yet very important re-scheduling I've mentioned would go a very long ways toward greatly improving our conventional-speed system--even when necessitating day-slower schedules cross-country to make it all work (which most logically would, incidentally making passenger service more mixable with frieght and not be greatly detrimental to the percentage of travelers that do currently go cross-country by rail)!
