There are many ways to do this trip. We took 5 weeks to go from Beijing via Mongolia to St Petersburg then Moscow. We stopped off at Irkutsk and Ekaterinburg for 2-3 nights each, and spent a week in each of St P. and Moscow. If you'd like to do only some of this trip, as the train from Ulan Bataar to Ekaterinburg is not very eventful, then a circular trip from say Moscow to St P. then Ekaterinburg and back to Moscow via the 'Golden Ring' towns/cities not far from Moscow. Or just the St P. - Moscow return then the 'golden Ring'. Feel free to ask any questions.