I think, most of foreigners will come to Belarus to the hockey championship by plane. But some, the european ones, will come by train or by car. There are a number of regular trains from Europe to (or through) Belarus. Maybe they will send some additional trains.
By the way, you may know, that the track gauge in Belarus (Russia and other ex-USSR countries) is different from european one, and all trains from Europe have to change their weels on the border.