I should add a couple of more things. When you get to the Civitavecchia train station you now have several options. You can buy a ticket to Rome as described above (I do not think there is a savings for round trip,,,but good to buy both ways to avoid the line and delay in Rome). But, there are two other tickets available at the train station. You can buy a ticket for about 12 Euros that includes the train plus a full days use of the buses and subways throughout Rome with the exception of the HOHO bus (hop on hop off) which I believe is bus # 110. There is also a ticket for about 22 Euros that includes the round trip train plus use of that HOHO bus (I am not positive about the #110 designation). You can ask about these tickets at the Civitavecchia train station.