Please see the very first post in this section for all the details you will need for train travel in Italy. It is very detailed.
Hank is (as always) spot on here as the BIRG ticket is the way to if you are going to and from Rome. If its just a one way ticket from Rome to Civ, then you can just get a one way train only ticket as that is all you will need.
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