we are planning a cruise out of Warnemunde. Can we fly to Paris and take a train to Berlin and on to Rostock? At the end of the cruise we plan to go back to Paris.
If you plan on flying into and out of Paris, you could indeed take the train from Paris to Berlin and on to Rostock. Check train schedules at www.bahn.com, and keep in mind that the route from Paris to Berlin requires reservations (if you plan on using a rail pass) or advanced ticket purchase (if you're purchasing point-to-point tickets). You can purchase your train tickets at http://ricksteves.raileurope.com/us/rail/point_to_point/triprequest.htm.
However, to save you both time and money, consider flying from Paris to Berlin/Berlin to Paris. Flight time is about an hour and a half, as opposed to an eight- or nine-hour train ride (or longer, depending on departure time). Flights can be considerably cheaper as well. Check www.skyscanner.com for flight options.
Additionally, as another poster mentioned regarding your flights to and from Europe, you might consider arriving in Berlin and departing from Paris (or vice versa), so as not to back track.
Bon voyage!