Originally Posted by
phillipas
You won't be taking the Trans-Mongolian from St. Petersburg - the train starts in Moscow!
It's a bit tough to give specific advice with the limited info you provide, and really you need to do some serious research and work out the options.
Some general thought:
1 - If you book the train part of the trip before departure you're a fool. You'll have no flexibility and any stops will be in theroetically-decent-but-in-reality-crap-and-overpriced hotels. Use an agency to get a Russian 'business' visa which means that you can simply roll up in Russia and have 30 (or whatever) days to do as you please (i.e. take the trains and stop where you see fit) with no need to have anything pre-booked. Both train tickets and hotels are easily sorted as you go.
Is this based on personal experience with buying Trans-xxx tickets in Russia?
While I agree that one is better off buying the ticket in Russia, your other advice about making stops wherever the OP sees fit is not exactly good advice unless things have changed since I took the train in 1998. Back then, the fare went up immensely due to taxes on domestic tickets.
What I would do is start in St Pb then take the overnight train to Moscow. I would then recommend two stops, at Irkutsk and at Ulan Bator. If necessary, buy only a Moscow to Ulan Bator ticket since the OP could easily spend a month traveling around Mongolia...then it's just a day's train ride to Beijing.