I'm not sure you can simply improve the current infrastructure and buy some Pendolino-type stock - many of the lines in Texas are actually single track, currently used by freight trains, presumably some of it coming up from Mexico. My guess would be there's a lot of bridges to widen, second lines to lay and a big electrification project. What do you do with the existing freight? Mixing very low speed freight on a line like that in the North East corridor would be difficult.
I'm not saying it would be free, or even cheap, to double/triple track the major texas routes and bring them up to modern mainline standard. But $15B is probably at least 3x what would be needed to put together a strong and competitive service. Even if the extremely-time-sensitive business market stayed with the airlines (as it has largely in the NYC-BOS market, for instance), picking up mid-to-high end leisure travel and less time-sensitive business travel would really reduce the burden on the airlines and airports. BA still flies from LHR to MAN, for instance, just a lot less frequently than they used to.
All-in-all I think there are probably better candidates for such a scheme in the US.
Could very well be true...California, the Midwest, Florida, North Carolina, Virginia, various branches of the Northeast Corridor, etc are all strong candidates for 100mph + railway operation. But if oil stays at these levels, we'll need solid regional rail service pretty much throughout the developed portions of America, including Texas.