A sleeper HSR train (250km/h) was just introduced for the Shenzhen-Beijing route 2-3 days ago.
If one isn't doing city-hopping for sightseeing, switching to this train at Changsha (after traveling from Kunming) will be very comfortable. That is, one can almost "sleep" all the way from JB to Moscow by train via this Changsha-Beijing route (except for the Laos & Yunan-Changsha section).