I did more or less a half of it—started in Moscow and got off in Krasnoyarsk. I took that trip alone, in the 3rd class (platskart in Russian) which are open space, without doors between compartments. I think for foreigners 3rd class long distance Russian trains are not only the most budget-friendly but are also a fantastic way to observe and interact with Russians of different ages and backgrounds.
Krasnoyarsk, the city where I got off the train which is about a 3 day train ride away from Moscow, is worth stopping in as it is surrounded by national parks, forests, mountains. As Natalia mentioned it, Baykal is definitely worth exploring as it’s one of the most popular tourist destinations of the country which can offer a lot, though I’ve never been there but it’s on my bucket list.