I've taken the TransSib (or TransMongolian, in this case) from Ulaanbaatar to Moscow. It's pretty much 4 days, just short of 100 hours. No showers, and the restaurant car was so-so; I mostly ate nuts and snacks: the train would stop some 10 minutes or so at stations, and there were kiosks on the platform to stock up. You really need to take some good books or good friends.

BTW, "The Man in Seat 61" has lots of info on it:

https://www.seat61.com/Trans-Siberian.htm