That's why I added the word "usually" in there. Try taking a train to Australia or Japan (from outside those countries), between the Americas and Europe or Asia, etc.

Generally anything separated by water should fly, but even just going across the continent I think flying wins out; eg why train from shanghai to Urumqi for 2-3 days when you can just fly for 5 or 6 hours?

Yeah, that's the point I was trying to make before with my first sentence. A 5-6 hour flight isn't that hard to do in a seat; you don't need a sleeper for that. But a 2-3 day trip on a train isn't something you want to do in a regular upright seat.