[beerandt]

It doesn't work like that- the geometric design of high speed rail and interstate highways are significantly different. The same is true for heavy rail vs high speed rail. You need much larger curves, and much smaller gradients for high speed rail.

Also clear space requirements for interstates would likely be infringed, which is one of the biggest factors of safety.

A more workable approach would be to reclaim abandoned rail ROW, and purchased additionally required ROW to extend curve length, where required. However much of this ROW has already been reclaimed by landowners and built upon.