I'd like to add to this that if you plan to take the Rapid Airport train from CTS to downtown Sapporo, pay the extra 300 yen for a reserved seat (especially during the Snow Festival), even if you have to wait 15-20 minutes for the next train to get a reserved seat. These trains can become extremely crowded and being in the reserved "U-Seat" car will allow you (limited) space for luggage - plus the train crew keeps the standees out (the reserved seats also have power ports).