i took the overnight train from beijing to shanghai last month. wanted to save money on a hotel and also try something different.
honestly it was okay but not amazing.
i booked a soft sleeper. that is the nicer option with four beds in a compartment. there is also a hard sleeper with six beds but my friend did that and said it was loud and uncomfortable.
the soft sleeper bed was fine. not luxury but fine. they gave me a pillow and blanket and a small bottle of water. the compartment had a door which helped with noise.
what i didnt expect was how early they wake you up. the attendant comes by like 45 minutes before the station to swap your ticket for a card. so dont plan to sleep in.
the train left around 8pm and arrived around 8am. timing worked well. i saved one night of hotel cost and didnt lose a full day to travel.
would i do it again? maybe. if i was on a tight budget yes. but if you can afford the high speed train which takes 4.5 hours just do that. much easier and less exhausting.