Why do you need a travel agent? If you just want to spend 7 days on a train, all you need is a train ticket - which you can probably get from ÖBB (you can definitiley buy it from Deutsche Bahn). Now if you want to get on and off the train and explore Russia a bit, staying a few days here and there, getting off the beaten track and so on - even this should be doable without a train agent

P.S. You do know a one-way ticket to Moscow on Niki from where you live, if bought in advance, would be cheaper than taking a train via Kiev or Minsk, do you?