wlm- probably too late for you, because I think you may have already left for your cruise, but Hank is correct. They run a shuttle bus from the ship to the market area in town. You can then take the 1R bus to the train station. We were off the ship before the shuttles had started running and managed to get a taxi for 20 Euros (big rip off but we had 4 couples in one taxi so it was only 5 euro each.) On the way back we took the bus (1 euro) to the shuttle drop off point. You want the 1R bus and you buy the tickets from the news stand shop inside the train station. We saw Florence and stopped in Pisa on the return and still got back to the ship with an hour to spare. Just watch your train travel time.
Carnival Cruise Lines:
Fantasy 90, Celebration 95, Victory 05, Conquest 05, Spirit 6/06, Freedom 6/08, Holiday 7/08, Victory 6/09, Valor 9/09, Dream 6/10, Splendor 8/10,Conquest 8/11,Magic 11/11,Triumph 4/12,Spirit 7/12,Freedom 11/12, Triumph 2/13 - The fire cruise
NCL Dream October 2006, RCCL Mariner OTS June 2007, Celebrity Silhouette 2013