And of course there are a lot of existing alignments in the US that are not fully built out that fill the bill for a medium high speed alignment

To which replied:

I would count only Washington-NYC(-Boston) and a few shorter stretches. I assumed upgrades to the existing lines, including cutoffs and tunnels, not simply different use.

I am not sure what "only count" applies to here.

Cleveland / Akron / Canton / Newark / Columbus / Dayton / Cincinatti / Louisville would be usefully served by 100mph rail. Many of those legs could run on existing alignments.

Detroit / Toledo / Cleveland / Buffalo / Rochester / Syracuse / Albany / Boston would be usefully served by 100mpg rail (ditto).

Miama / Fort Lauterdale / Orlando / Jacksonville and Atlanta / Chattanooga / Nashville / Memphis, especially with a VHS Dallas / Memphis / Atlanta / Jacksonville.

I've been accused of being a Marxist, yet while Harpo's my favourite, it's Groucho I'm always quoting. Odd, that.