Transsiberian

You should be fine, though allow time for delays. Since it's off season you shouldn't have a problem buying tickets for the next day or same day. I did it in March and was fine buying on the road -- much cheaper, too. You just might not be able to always get 3rd class bottom bunks, if that's what you're going for. The shorter your route, the more availability of next or same-day tickets.

Thank you! Can I also ask you how cheap is this trip possible to be? (just talking about trains)

Do a forum search. I wrote a trip report on the forum, which can be viewed here. The report lists exact train costs. Where 'kupe' is listed (2nd class), platzkart (3rd class) will be around half the price.

Hey guys ,I'm doing the same trip early Jan, thanks for the link Muteki, very helpful.

So it is easy enough to buy a ticket as Irkutsk station to get to Ulan-Bator, and then to Beijing? Just wondering as I can buy tickets on the official Russian rail website for internal russian trains but not into Mongolia etc…
Thanks.

I think you'll be fine. Were you on the English version of rzd.ru? I found that part is missing a lot. On the Russian side it lets me get to the purchase section, though it's notorious for not taking foreign cards.

http://pass.rzd.ru/timetable/public/en?STRUCTURE_ID=735&layer_id=5354&refererVpId=1&refererPageId=704&refererLayerId=4065

That's what you're looking for. Thanks God, russian railways translated the whole booking system, though it does have its moments.

Check all your train legs there. Choose Reserved seats (strange translation) for 3rd class. There are common seats, but take it only if you REALLY count every penny.

Hello!!
im doing the same trim in april.
may i ask, what agency are you usiind for tickets? have you heard of visitrusia.com ?
im using them i just want to make sure they are legitimate.

thanks! how are the common seats?
i'm thinking about just buying the ticket in the station, do you think it will be a problem in january? i can't know beforehand when exactly i will be in moscow, so i can't buy through an agency...

i really don't have an idea. this will be my first time in Russia.
i am ussing an agency because it makes it easier to trait the visa...
have a great time!!!