Our plan is to take the train and I was a little hesitant at first, but I searched on
YouTube and found some videos some folks took of the bus to the train station and the train station itself and it all looks pretty straightforward. We disembark in Civitavecchia so we'll have luggage as well, but it still looks fairly simple.
I'm going to paste my (probably overly extensive) notes I took below. Again, we haven't done this, but hopefully my notes help a little in case you decide to go this route. My notes are specifically to get to the Colosseum (which is where our accommodations are) so if you're going elsewhere, you may need to make a different note of the line you want to take once you get into Rome.
Official Map of Civitavecchia port:
https://civitavecchia.portmobility....p_of_port_of_civitavecchia_eng_1465288193.jpg
Take a cruise port shuttle bus to the Service Center of Largo della Pace, then walk (or take a bus for €2 to Civitavecchia Station). Walk is about 15-20 minutes. Looks like the €2 bus runs about every 10 minutes so it’s probably worth saving our feet and paying the €6 to take the city bus to the train station. It looks very simple.
Leaving from Civitavecchia Train Station you can get to the Colosseum in 1 hour and 10 minutes. Get off at Roma Ostiense Station, go on foot to metro B line at Piramide station. Take the train for Rebibbia and get off at stop Colosseo. The IC trains are non stop as opposed to RV which stop and take longer.
If there is a line for tickets at the ticket windows/self serve machines, the little convenience store inside also sells tickets. A BIRG ticket will be good also within Rome for the rest of the day so that may be the best option. Some of the express trains to Rome don’t take BIRG however so double check.
Make sure to validate the train ticket prior to getting on board.
Express train timetables:
http://www.trenitalia.com/tcom-en
All timetables:
http://traintorome.maxonthenet.altervista.org/?lang=eng
More info:
https://civitavecchia.portmobility.it/en/port-civitavecchia-guide-cruise-passengers