from San Pietro station to Ostia antica

Take a regional train from Roma San Pietro station to Roma Ostiense station (about 10 minutes) where you'll switch to the local train to Ostia Antica. Local train is covered by a Metro ticket.

is this regional train included with the Roma pass?

The Roma pass website says
"Free admission to ATAC urban buses and trans, Underground lines A and B, Met.Ro trains: Roma - Lido, Roma - Viterbo (in the Roma - Sacrofano section), Roma - Pantano railways lines."

but I don't know what line the San Pietro station is on.

thanks
kath

RS book shows The Cipro Musei Vaticani stop on Line A, followed by the Ottavaino (Vatican Area St Peters) so I'd assume Line A. I'm sure the innskeepers can help you. Our hotel had a free map with the transit system on it that was good. Knowing Google Map you may find the answer there

Kath - I'm assuming you're staying near the San Pietro train station and not the Metro station. The Roma Pass would cover the Ostiense-Ostia Antica leg as that's on the Roma-Lido line, but it appears you'd need to purchase the Roma&Piu pass for the short San Pietro-Ostiense trip on the regional line to be covered. If it turns out you're staying near the Metro station instead of the train station, then you could do a combination Metro/Roma-Lido train trip that would be covered by the Roma Pass.