On www.trenitalia,com click the Italian flag to find English, enter your 'hour' of departure e.g. enter '9' for 09.00, and enter Roma S. Pietro to Tarquinia. To see fares, use a date within the next seven days. Buy tickets in Italy and validate (time/date-stamp) before boarding.
Note that Roma S. Pietro train station is not in the same area as the Ottaviano San Pietro Metro station.
Where do you want to end up in Tarquinia? The train station is down in the valley near the port, which requires a bus to get up to the historic center of town
You can either take the train to Civitavecchia and get a bus from there, or to Tarquinia and get a bus from there. Once in Tarquinia it's easy to walk between the museum and the necropolis as they are less than a half mile apart.
Tarquinia has a decent tourism web site in English: tarquiniaturismo.it/pagina14_city-of-tarquin…Edited: 04 April 2015, 13:04
I just went to Tarquinia at the beginning of March from Rome. We got on the train at Trastevere station (near where we were staying). We bought tickets to come back at the same time which you can do since it is a regionale train with no fixed seating. Trastevere station is not manned after 2 pm or on weekends, so you have to buy the tickets from a machine it you go when there is not a person there. The Tarquinia station is unmanned period and you would have to buy tickets from a machine if you wait till you come back I get nervous myself about the machine not working/being broken. Last time before this when I was at Trastevere station only one of the ticket machines was working. Not sure what San Pietro is like?
When you get to Tarquina, you go to the front of the station and there are 2 bus lines that can take you into the city. You pay the driver for tickets so have some coins/small bills on you.
The bus leaves you off in front of the Museum. The people inside will help you with a map and how to get to the Necropolis. it is a pleasant walk to there. The tombs were awesome and there was a snack bar open when we were there.
Thankyou all for the valuable assistance,
Board the train at Roma Termini going to Tarquinia, rather than at Roma Trastevere The rest of the posting remains the same. Note that the Hotel San Marco in Tarquinia (mentioned in the hyperlinked posting) shut down two or three years ago. However, there are excellent bed and breakfast nearby, including the wonderful Palazzo Rosati b&b across the street from the Archeological Museum.