In China, if the train you want to take is full such that no tickets are available, you can buy a short ticket and supplement it on board by paying the fare of the remaining section plus a small fee. This causes the train to be overloaded which generally isn't a problem except that you don't get a seat on the supplemented section, unless the train is the newest model train which isn't designed to be overloaded.
However, a recent discussion on Flyertalk suggest that this won't work on the trans-Siberian railway.
The poster needed to travel eastward between two cities along the route and all beds in the eastern direction have been sold out, and he claimed that he won't be allowed to board the train when all beds have been sold out.
He also claimed that, given the vast distance between the cities (about 2000 km), using buses, taxis or hitchhiking aren't practical, and he couldn't find any flights connection anywhere on the route, and he ended up taking a long-haul flight to Europe, and another long-haul flight back to Asia.
Therefore, I need to avoid putting myself in such situation if I need to travel on the trans-Siberian railway. My question is that, if I need to travel on the railway and find that all trains have been sold out in the direction I'm travelling and there is no alternative transport available, what method can I use to board the train while paying the train fare on board?