> 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.