VISA-Information for Trans-Siberian-Railway-Travel

Dear Ladies and Gentlemen

I suppose to travel with the Trans- Siberian- Railway from Asia back to Europe (from Beijing to Moscow).
this would be around mid-July 2014.
How dos it work with the VISA? do i need first apply for it, or do i have to buy first the train-tiket?

I arrive next week on the 5th of May in Bangkok and would you pay a visit. However, since I can only stay a week in Bangkok, I am much obliged and grateful for constructive information in advance.

With best regards
Andrea Mueller

(Switzerland)