Really? I was thinking about it, but considered -25°C too cold for my likings. I mean, what do you do in the cities? Walking around at -25°C? :(
It easily gets to -40°C in Siberia.
Supposedly the landscape is beautiful in the winter, thanks to the snow. Personally I chose to see it in the summer, though, so I can pack fewer clothes.
Usually it’s quite warm inside the train, unless you’re unlucky to get a broken car.
I heard from someone who did it in the summer that to do that is missing the point, since at that point it's just a big forest and not very, well, Siberian.
(I went in Spring when it was -10, which I think was about the right mix: endless snowfields, and plenty cold enough).