[ChiSoxRox]

Madrid to Barcelona is 313 miles straight line.

Milan to Rome is 297.

Both corridors used to be air dominated (and amongst the top 10 corridors worldwide) and now are ~80% rail dominated.

While a national Shinkansen doesn't make economic sense, there's plenty of those natural high speed rail corridors in the US, beyond those with current projects:

Seattle to Portland
Boulder to Pueblo (even Albuquerque?)
Chicago to Minneapolis, St. Louis, Detroit
Chicago to and in between the Ohio Cs
St. Louis to Kansas City
Atlanta-Greenville-Charlotte-Raleigh
Philadelphia to Pittsburgh
The Empire Service as HSR
Actual HSR on the Northeast Corridor
Memphis-Nashville-Knoxville

Let's make regional flights (<1 hr) a thing of the past, aside from islands.