I'm an Eastern European and I really love train travel. I've traveled extensively in the region both on rail and on airlines and I cannot emphasize enough how much I would advise you NOT to take the train.
Yes, I know many who have had pleasant enough experiences on the local trains (generally within a given country is better than international) and I myself have had some great fun on trains from Germany to the Czech Republic and Hungary.
Having said this, Eastern European trains are slow, uncomfortable and can be dangerous on certain routes. The route you mention is subject to frequent delays, bad food and the whims of the authorities. There are frequent robberies (yes the gas stories are true) and especially in Belarus, you are not really in a position to complain with the railroads or local police since one or both are usually in on it. If you choose to do it, and do run into trouble, DO NOT PUSH the situation with local police or you are in for a rough few days in a local lock up. If you really must to do it, go with a local or groups of locals, dress down and interact and have fun with as many local people as possible.
You can now fly a variety of airlines in the region and the time saved and risk avoided is truly worth it.