Actually, that existing track is part of the problem, not the solution. A new high-speed train is much easier to justify economically when you're looking at (total benefit of high-speed rail) - (cost of high speed rail) than when you're looking at ((total benefit of high-speed rail) - (total benefit of current rail)) - ((cost of high-speed rail) + (cost of removing current rail if necessary)).
There's a name for this effect, but I can't remember it. Another example in the real world is the effectiveness of putting up cell towers in developing countries rather than running wires for landlines.
Perhaps you're referring to the term "leapfrogging", the topic of which Kevin Kelly offers a differing viewpoint from the mainstream acceptance despite explaining this in 2006[1].
Even for US high speed rail, I don't think any of those trains will be designed by an American company.
From Wikipedia: In February 2015 nine companies formally expressed interest in producing trainsets for the system: Alstom, AnsaldoBreda (now known as Hitachi Rail Italy), Bombardier Transportation, CSR [China South Rail], Hyundai Rotem, Kawasaki Rail Car, Siemens, Sun Group U.S.A. partnered with CNR Tangshan, and Talgo.[24]
Even though they might be foreign companies, they do tend to be built in the US. IIRC Alstom in California made all the cars for the new Florida high speed line that starts running this year.
It takes a while before you can start building rolling stock domestically. Melbourne just started making their class E trams a few years back, which they tout as being "made entirely in Victoria" and yet they've had the largest tram network in the work for decades.
Then I remind people that they have 20,000km of track, 150 trains and how much operating experience? That's a huge head start.