We already have an antique low speed rail network that covers most of the US population centers. That makes it even cheaper to upgrade to high speed rail, since most of the land has been bought.

Also, when people in the US say "high speed rail", they usually seem to mean 180mph. That does not qualify as high speed rail overseas.

Upgrading the current low speed system to medium speed 180mph rail would mean that taking the train would actually be faster than driving (and for short air routes, faster than airport+flying).

My guess is that many existing routes would then become financially viable.