I regularly take the train between Moscow and Kazan. It's 12 hours, you can get on in the evening, have dinner on the train and then get a decent 8 hours of sleep before getting up, having some coffee and arriving.

It's much longer than the equivalent flight, but also much more comfortable. There's something annoying about airports - with the train I can get to the station 15-20 minutes before departure and it's fine.

Once the train rides get much longer than 12 hours it shifts, but there's a sweet spot right around there.