I once took an overnight train from Paris to Berlin, arrived at about 7:00 a.m., and walked clear across the city to my AirBnb (from Mitte to south Neukölln, for any Berliners.) It was a really fantastic way to discover the city for the first time. I don’t think flying in to an airport then taking a taxi would have been nearly as memorable.
The only problem IMO is that sleeping on a train is not comfortable unless you’re young. It’s basically a minor step up from staying in a hostel in terms of noise, other people, and general cleanliness. I think there would need to be a dramatic upgrade in the quality of sleeping cabins for trains to really replace flights.
ÖBB (Austrian Federal Railways), who are slowly taking over night train service all over Europe, have single or double Deluxe cabins [1]. You can have your privacy and your own bathroom with shower. It can be pretty private and comfortable.
i've had a Deluxe cabin experience with the shower. it was wonderful and private but anecdotally i still ended up sleeping better in the first class seats of the TGV that we had connection to right after the night train
One problem with AirBnBs and sometimes hotels is that they're not really set up to receive new visitors early in the morning, even if you only want to drop off your bags.
As for sleeping quality on the train: I'm looking forward to the new one-person ÖBB cabins.
The only problem IMO is that sleeping on a train is not comfortable unless you’re young. It’s basically a minor step up from staying in a hostel in terms of noise, other people, and general cleanliness. I think there would need to be a dramatic upgrade in the quality of sleeping cabins for trains to really replace flights.