We are planning on doing exactly what the headline says - is it even possible? We are planning on going from Moscow to Beijing with stopovers, maybe 4 or so before hitting UB where we'll get off for min. 2 weeks. Probably leaving around the 1st of July (depend on exam dates). We would rather not be hung up on a strict schedule, and just go as we like. We are not really in a hurry, if we have to wait a day or two for tickets somewhere that is fine. We are fine with sharing compartments - doesn't have to be fancy in any way :-)
So do you guys think it is doable? or will ALL the tickets be completely sold out? We really hope you can give us some pointers and thank you so much in advance!
Lærke and Martin

Everbrite is usually the exeprt on Trans-sib, I@'ve only done small bits of it, but here's what I found.
There are many trains covering different parts of the journey. The low numbered trains are more luxurious, make fewer stops and travel longer distances. They also sell out first. So if you delay purchase you will be on the less comfortable trains, perhaps even struggle to get a bunk for the night and be in open-plan platskartny instead. The trains also thin out as you go further east, so you risk being stuck in Ulan Ude or Irkutsk for weeks on end.

In my experience it's normally easy to get tickets a couple of days in advance for all stretches of the transib within Russia. This is especially so if you are making a few stops along the way, as the shorter the distance, the more straight-through train options there are. I usually go kupe (2nd class), but even if you go platskart (3rd class) you always get a bunk to sleep on on overnight trains. (i.e. if you have a ticket, you have a bunk.) The differences between low and high number trains are generally fairly cosmetic, unless you want to go SV/liuks (1st class - 2 berths in a compartment), which doesn't exist on a lot of high numbered trains.
However, apparently demand exceeds supply on the actual cross border bit (Irkutsk - UB), so people recommend organising that in advance (especially as that's obviously the point where your visa expiry date may be an issue).

As above, I've never crossed the border, but I've travelled various sections of it a fair bit and have never booked a kupe ticket more than one day beforehand - and I've never had a problem. On plenty of occasions I have also bought same-day tickets at the station, usually in the height of summer. So my impression is that the internal sections really shouldn't present any difficulties, but it sounds as though you might have to plan for the international leg.
Generally speaking you can get a ticket in summer with a day or two if you are flexible about the destination. You might have to make an intermediate stop or not take the train at the most convenient time, but you will be able to leave.
The choke point as noted is crossing the border. There is only one train daily that crosses from Russia to Mongolia and it has only one kupe car with 36 berths that actually cross. Because the crossing takes such a long time, it is possible to take this train to the border in a wagon that does not cross, disembark, hire a taxi and then rejoin the train in a local wagon in Mongolia or it is possible to take the bus from Ulan Ude to Ulan Bator.
The one caveat to all this is that if you are trying to get to UB to attend Nadaam from July 11-13 you must book your tickets in advance as in the past it has been very difficult to get tickets last minute to arrive at this time.
Also due to the specifics of the visa, assuming that you have a tourist visa for 30 days, you might want to consider buying your final ticket out of Russia when you arrive. If you want to take the train that travels from Irkutsk (or Ulan Ude) to Ulan Bator, tickets for this train go on sale 45 days before departure and can be bought at any train station. I would suggest buying this ticket when you arrive in Russia as you will know from you visa or general plans by when you must leave. Overstaying can be unpleasant and expensive.
Ruth