There is a good mainline train service between Civitavecchia and Rome and when I was there on a previous cruise, many people made the trip themselves. Have a look at a Thos. Cook railway timetable - your travel agent should be able to help.
As an alternative, I can recommend nearby Tarquinia.
If you are familiar with Rome, or speak some Italian, or are comfortable about using those great European trains, go for it. You can really save.
Hi...you can take the train from the port...about a 5 minute walk...to Rome. Once in Rome, you can meet with your tour guide. Private tours are more expensive, but well worth it. If you can get a group of about 6-8 that would be great because it's small and you can share the costs. Go on the web and search for Rome Day Excursions, you'll see plenty of them all varying in costs. The train schedule can be found and tickets can be reserved at www.trenitalia.com Cost is cheap! firstname.lastname@example.org is a good contact...highly educated American guide. Hope this helps!!!