There are two trams of interest that go through Trastevere.
No. 8 goes north across the river to Largo Argentina...near Campo di Fiori, Pantheon, etc.
No. 3 turns east across the river, then meanders toward the forum, colosseum and San Giovanni in Laterano. [Note...for reasons I never discovered, this "tram" line was actually being serviced by buses when we were there this past December...probably a track maintenance issue.]
Both trams go south to the Trastevere train station, which has the FR1 train to/from FCO -- a 26 min ride for 5.50 euros (December...may be more this summer). See the train map at: http://www.atac.roma.it/docunet/file.asp?mid=3&rid=385


