Well aware of the distinction between HSR and HrSR. How do you come to the conclusion that the Capitol Limited is the only LD train that would benefit from high speed rail? I guess I don't understand what distinction you are making between the CL and all other LD trains.
If a train has to go overnight, with large amounts of daylight on the day of departure or arrival (even with a conversion to high speed rail), it doesn't really make much sense (at least to me) to make it so. If the idea of high speed rail is to speed up trips, it makes better sense to use the technology on routes where the travel time would be cut from ~8 hours to 3 or 4 hours.
At the end of the day, a long distance train will never be able to beat air travel in terms of time traveled. Even if a train can go 220 MPH, an airplane is still going ~500 MPH. The reason why the Acela does so well is that it offers faster point to point travel than the airlines can (between WAS and NYP). Even though a plane is technically faster, when the fixed time period between getting to the gate, and getting from the gate to your final destination is factored in, the Acela wins due to the relatively short distance between DC and NYC. It can't win though if you are traveling between BOS and WAS, because the total travel time by plane can still beat the total travel time by rail.
If a high speed train still can't beat an airplane for total travel time (including ground transportation, security, etc) it doesn't really make sense to upgrade it to high speed status. It would be a better application of limited resources to make all short and medium distance trains high speed, and upgrade long distance trains to increase reliability, with some smaller speed increases where the benefit is appreciable.
I would personally love to see every train be high speed, but one must consider the benefits (boost in ridership, etc) gained from spending the required hundreds of billions of dollars. High speed conversion carries a very high cost per mile, and just doesn't seem cost effective on routes as long as the ones that go from Chicago to the west coast.
The reason I singled out the Capitol Limited is that (catch me if I'm wrong) it is the shortest and more direct LD train in the entire system. It would seem to me that it would be the best candidate if we were to upgrade a LD to high speed. It could either take the form of a day train, or a later evening to earlier morning train. The latter option could be billed as a combined hotel/transportation means to business travelers. As it stands, the departure time from DC (4PM) and the arrival time into CHI (9AM) are too early and too late, respectively, to be able to market the train in this way. If the departure time could be made 7 or 8 PM, with an arrival time of 6 or 7 AM, the high speed upgrade would be worth it, and with the right business plan, the route could become very successful.