Though I wasn't fortunate enough to make it into Beijing via Moscow on the Trans-Siberian route, I was able to take the train from UB to BJ on the Trans-Mongolian line. I had read about how it was a social train and it sounded like many other like minded travelers would be on it and endless fun would ensue. You may have already guessed this, but this was not the case. Passengers mainly stayed in their rooms and the cafe car that I had read was a hotbed of social interaction was actually a restaurant car where people got food but did not really communicate. I did meet a few other travelers who had had similar thoughts and were surprised at the sterile like environment of the train.
I had wanted to go 2nd class to meet more people but all I could get was 1st class. I know, not the end of the world. My roommate was a turkish fellow who had travelled over 80 countries. He didn't like planes so mostly took trains, boats, buses, and cars. He was very friendly, though he slept for the majority of the 30 hour commute.
My car attendants checking the people in line ahead of me. I had someone take a picture of me with the ladies, but they ended up taking a video instead. Not sure why but this "asking for photo and getting video" was a common thing in mongolia. odd