Since I am working on one of these things, I have to be fairly careful in what I say as if it has not already been released to the public, I cannot say it, and I won't, so don't bother asking for any "inside" information.
25kV AC at "coffee pot" frequency, that is 60 cycles/second in the US, is being treated as a given. That is what was used on the New Haven to Boston, and is used by all other major high speed systems in the world. Exception: I think the Italian Directissima may use 3kV DC, because that is what is the standard in Italy, but would have to look it up to be sure. Overhead wire constant tension catenary also. Any serious study on this issues is most unlikely.
Of the equipment sets under consideration, the widest is the Japanese Shinkansen sets, which are 3380 mm wide. That is 11'-1". This is slightly wider than the standard AAR "Plates" which are 10'-8" wide. The European equipment is somewhat narrower. There was a report on these varios types of equipment that appeared in an AREA Bulletin something like 20 years ago. These basics have not changed since.
The standard height limit for the Japanese equipment outline is 4500 mm, and the standard wire height they use is 5100 mm. Gap is 600 mm between maximum vehicle height and wire height. With slight rounding, these are 14'-9", 16'-9", 2'-0". There is a set of European clearance gauges which all equipment used in run-throughs is Western Europe must clear. The highest of these is about the same as the Shinkansen limit. These facts should be a hint that the high speed vehicles will not be as tall as a superliner, although there is no particular reason that make is either impractical or impossible.
Unless a higher overhead wire height is used, the Superliners cannot run on the high speed tracks as the electrical clearance would only be about 6 inches, and even if acceptable in theory (I don't know), would you want to have a big chunk of metal filled with people within 6 inches of a 25 kv bare wire?
There are turnout designs for 100, 130, 160, and 220 km/h (call it 60, 80, 100, 135 mph) found in German standards. Not to say that these would be used exactly, but that such designs exist.
There are FRA rules about what traffic can be run on tracks that are cleared for very high speeds.