One thing that has been pointed out is that besides going to Vladivostok, there is a weekly train that runs to/from Beijing. I would find that quite interesting. First class fare I saw for the trip was about $1200 [for a room for two], and I believe meals are extra.
Many tour operators will price for this however it can booked very easily and cheaper direct, also there is not 1 but 2 weekly trains between Moscow and Beijing- one passes thru Mongolia and the other goes around to avoid the additional visa.
Further to this if you are breaking your journey on top the direct services there is additional trains between Irkutsk and Ullan Bator or Ullan Bator and Beijing meaning if you wished to break your journey in UB (and I'd recommend this) you can without having to wait a week to continue your on ward journey.