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)