For any person not a citizen either of Belarus, or of other former Soviet countries -- it is forbidden to cross Belarus by train, without a transit visa. In general, such visas must be obtained outside of Belarus (you can't get them at the border), and the process takes several days.

The most direct route from Moscow to Berlin passes through Belarus.

Travellers can avoid this obstacle, by booking their rail trip by way of Riga (Latvia). This route is longer, but bypasses Belarus.

P.S. Learned about this the hard way -- was turned back at the first station after crossing the frontier in 2004. The only time I've been ejected from a country (so far!).