Hi there, every time we call into Civitavecchia we hit the train in to Rome, it is unbelievably easy. The trains run every half hour or so (see http://www.trenitalia.com/en/index.html
), and the ticket office speak great English - they usually even have the tickets ready for the cruise passengers.
P&O & Princess (or the port authority) usually provide a free shuttle bus to the dock gates, if you are moored on the outer sea wall, it is also an easy walk around. From the dock gates, walk for about 10 minutes keeping the sea on your right hand side, you will see a wall rising on the left side of the road, cross here & walk into the car park there, which is for the train station. After buying your ticket, you must clip/chad it using one of the machines by the platform doors.
There are 5 stations in Rome, the first you come to is St. Pietro which takes you into St. Peters Square & the Vatican, the next 3 are subrurbs of Rome, the last is Termini where you can catch a hop on / hop off Tour Bus.
Don't bother booking a seat, most of the ship will pack onto the train, so you've got little chance of finding the seat, let alone sitting on it.
The train journey lasts about and hour & 15, depending on the number of stops en route. Make a note of the station names, so that on the way back you are ready to get off.
If you are heading back to the airport, be aware that there are 2 in Rome, and only one is easy to get to using the trains change at Termini is easiest & thgen back out on the Ostia Lido line.
have a great time
Alan & Katrina