I'm visiting Rome on a cruise in July and would like to book my own transportation from Civitiavecchia to Rome and back. Looks like the BIRG ticket for the train is the best option. Do you know how to book the train and buy the tickets ahead? I've been looking at the ATAC and Trenitalia and can't seem to find a way to do it. I'm concerned if I don't book ahead I won't make it on the train.
" I'm concerned if I don't book ahead I won't make it on the train."
If that was a real concern than trains would not be useful, and people would not rely on them for their every day transportation needs. Anyway, these are regional trains, and I would advise you not to book this in advance, as Trenitalia has made electronic train tickets for regional trains quite complicated. Just buy paper tickets.
It's only a few Euros, and the seats are unreserved. This is a frequent, basic, mostly local service, and not even like the Airport Express. Worry more about getting from the boat to the train.
Use the Search box top left to read about typical rushed one-day itineraries, and the risk of missing the ship departure. What time will your cabin level be able to get off the ship?
One example of many, many. Age of post not important in your context:
https://community.ricksteves.com/travel-forum/italy/rome-tour-in-a-day-from-cruise-port
Listen to Tim.
Also, provide us with more information about your trip. Have you been to Europe or Italy before? What kind of cruise is this?
You just buy the 5 zone BIRG at the station- https://www.atac.roma.it/en/tickets-and-passes/birg
Assuming it runs again this year, apart from the regular commuter trains there is a special cruise express train, which is an all seated and all reserved train- https://civitavecchia.portmobility.it/en/new-civitavecchia-express-non-stop-train-cruise-passengers
As a special service this is not covered by the BIRG ticket.