I mean, the entire US other than the NE corridor, California, and 1 or 2 lines out of Chicago. Besides that small handful, the Amtrak service that's there now sucks so much it may as well not exist.
But in the spirit of OP's question, I guess we should assume any existing rail connection counts as "already connected." So let's look at the Amtrak system map and see where there are holes:
chumwa on wikipedia
Obvious holes:- New York-Scranton-Binghampton
- Clevaland-Columbus-Dayton-Cincinnati-Louisville
- Jacksonville-Atlanta-Chattanooga-Nashville-Saint Louis
- Atlanta-Chattanooga-Knoxville-Kingsport-Roanoke-DC
- Richmond-Roanoke OR Richmond-Charlottesville
- Tampa-Sarasota-Naples
- Dallas-Houston
- Oklahoma City-Tulsa-Kansas City
- Milwaukee-Madison
- Detroit-Flint-Lansing-Grand Rapids
- Quincy-Saint Louis
- Atlanta-Tuscaloosa-Jackson-Dallas
- Denver-Colorado Springs-Albuquerque-El Paso
- Los Angeles-Las Vegas
- Los Angeles-Bakersfield