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.
Go ahead and make your case to Goldman Sachs. They've got plenty of money and connections to put it to use. I'll say that $5 Billion these days buys so little infrastructure it's not even funny. 100mph rail will attract few business travelers on a route >250 miles; leisure travelers are exquisitely price sensitive. It's 'Go fast or go home' in the rail business: even the cross-country freight guys have figured that out and are aggressively pushing time-defined express services.
There's little use for
islands of rail service - a stretch in Florida, a triangle in Texas, Detroit-Chicago-Milwaukee... Look at the highest passenger volume air routes today and few are viable for high speed rail, even before trying to recover capital costs. Build a runway and a 737 can go to over 400 points in the U.S. with scheduled commercial service today. A high speed train beween Tampa and Miami serves those points... and nothing in between.