We have taken that train many times in both directions. There is usually one train per hour (sometimes more) and the journey takes a little over an hour when using the regional trains. You can buy a special ticket called a BIRG ticket for 9 Euros. This ticket covers round trip to Rome on the regional trains plus unlimited use of the buses and metro within Rome. We use the BIRG when we have a port day in Civitavecchia. If you are heading to Civitavecchia with your luggage, keep in mind that the track used for the Civitavecchia train is often a long walk from the Termini entrance. Once you get to Civitavecchia you have a 4-5 block walk (its pretty easy and flat) to the port entrance where you can catch the free shuttle bus to your ship.