Between Rostock, Berlin and Leipzig runs the private operated train Interconnex. You can buy online a oneway ticket for 12 € from/to Berlin and for 21 € for the whole trip from Rostock to Leipzig or v.v., unfortunately their webside and their booking-side are in German language only. Tickets are also available at several travel agencies, but they are a little bit more expensive (17 € / 29.50 €). Another option is to buy the ticket on the train (20 € / 35 €). But note that you cannot buy the tickets for this train at the German Railways ticket offices. Also tickets from German Railways are not valid in the train.
