I recently travelled from Beijing to St Petersburg on the trans Mongolian rail. The trip was fantastic, but I had a very bad experience with the London based travel agency, Real Russia (www.realrussia.co.uk).
They tell you that they charge a commission of 15-20% of the cost of the train ticket. What they don't tell you is that they use local agencies to actually book the tickets who also charge a similar mark up. In my case I paid a total commission of 45% of the cost of the tickets had I booked them directly. The commission alone that we paid for tickets for 2 people was in excess of ÃÂ£500 and it definitely was not worth it! As we were travelling at the time, we had to go to the local agencies to pick up the tickets ourselves (not in convenient or well signed locations), and some of the information provided to us from Real Russia was wrong, leading to us not being aware we were spending an additional night on the train, which meant we booked a night in a hotel which we did not need, and we didn't have enough food on the train for the journey.
Real Russia are systematically misleading customers as to the commission they will pay, even when explicitly asked what the mark up on the face value of the ticket is, they will not give an accurate answer. The response I received from their complaints team after reviewing the transcript of the call was 'I can only apologise for the mis-information you received from members of our team, our team are not always fully aware of the costs for tickets'. I have spoken to the customer service team since making this complaint and they are still providing the same misinformation.
I would recommend booking through a reputable local agency (or directly online if you don't mind the trouble of translating webpages). Seat61.com gives some great information about how to book tickets. They recommend Real Russia highly but note that they do also get commission for any bookings made through RealRussia...