Border crossings can take several hours, with a wait of up to four hours on both sides of the border.
One reason for the long wait is to give the border guards the chance to go through their routine. Several uniformed border guards will board the train and begin to do two things. One group will go from compartment to compartment checking documentation, making sure that visas and passports are all ok, and that you match the photo in your passport. These guards will also check any relevant customs declarations are completed and handed over. The second group will check bags and compartments to ensure that no illegal contraband is being carried; you may even find them unscrewing the various panels in each compartment to make sure nothing has been hidden behind them! During these checks, you will be asked to stay in your compartment, and remain seated. This process happens on both sides of the border.
Once the border checks have been carried out, you will be allowed to leave the train. If you do, make sure that you take any valuables with you, and that you have what you need for the next hour or two. The reason for this is that now the train will move so that new carriages can be added, or old ones removed; for instance, the dining car is changed so that in Russia you have a Russian one, and in Mongolia, a Mongolian one. It may seem a little disconcerting when the train moves off without you, but it will come back. This can happen three or four times. When the train leaves for the final time, the provodnitsa will make sure you know so you are not left behind. Again, this process happens on both sides of the border.