It's about 1500 miles from one end to the other, and not exactly "dense".
Sure it's small compared with Russia or Canada, but it's larger than most.
Realistically anything upto about 500 miles shouldn't need flights -- that's 2-3 hours on a train.
Once you go over 1000 miles then train stops making sense, it's 5 or 6 hours compared with 2 on a plane. 5 hours might work for an evening meal/relax, departing say 4pm and hotel by 10pm. With decent wifi and power you could probably push it to 7 or 8 hours, so maybe 1500 miles if it was high speed all the way.
A sleeper train (which probably wouldn't be high speed) would work at about 1000 miles in 12 hours, leave at 9PM after dinner, getting you there for shower, breakfast, and a 9AM meeting. Would beat a 6AM flight (so up at 4:30)
Agreed. When the option exists, it's so much nicer than flying.
At one point in my career I traveled from Amsterdam to various other European cities for 3-5 day work trips fairly frequently. Where direct overnight train connections existed, I would try and use those rather than get up for a 6am flight. For example Amsterdam - Copenhagen used to exist as an direct overnight connection. The cost for a private sleeper cabin with shower and breakfast was, I think, about €500 -- less than a flight + an extra hotel night. The trouble was getting my company to reimburse it, because it didn't fit neatly in their travel reimbursement policy's categories and rules.
These city groups would also work with French style TGVs
Miami to Orlando, Jacksonville, Atlanta
Seatle/Portland/Vancouver
SF/LA/San Diego/Vegas/Pheonix
With sleeper trains you can connect New York to Miami in 12 hours, LA to Seattle, Chicago to New Orleans. You need the trains to run at about 110mph, which is normal local line speed.