I am really struggling with this section (and it's the first of my travels)! we are staying just outside San gimiano, and fly into rome (from aus) at 7.30am. we don't want to drive after travelling that far, or if we were to drive, get a car on the out skirts of town so we aren't in the midst of it????? so train or bus, I am aware I need to change at poggibonsi? thanks heaps for your help I'm really frustrated!
Leonardo Express train leaving the Fiumicino Airport at 08:08 - arrive at Roma Termini at 08:40.
Change train at Roma Termini departing at 09:20 for Firenze (Florence) Santa Maria Novella station (high speed Frecciarossa train). You will arrive at Firenze SMN at 10:51.
At Florence SMN change to Regionale (commuter) train departing at 11:10 and arriving at Poggibonsi-San Gimignano at 12:14. From the Poggibonsi to San Gimignano proper it's a short bus ride. I don't know where you are exactly located outside San Gimignano. If outside town, it might be best taking a taxi directly from the Poggibonsi station.
If the above doesn't work because your airplane is late (or your luggage takes forever at the baggage claim) there is the same sequence of trains exactly one hour later. Just add one hour to that times above.
However, considering that you are staying in a very rural area outside a rather small town (San Gimignano) and that therefore you will eventually need a car where you are staying, and considering that there are no rental car companies in San Gimignano (only in Florence or Siena), I would STRONGLY recommend that you drink a gallon of espresso coffee upon landing, rent a car at the airport, and drive straight to San Gimignano. The drive from the Rome Fiumicino airport is almost totally freeway (except for the last few miles from Poggibonsi to SG) and shouldn't take much longer than 3 hours (plus pit stops to drink more espresso). The freeway entrance is right at the airport and there is no need to drive inside Rome (early in the morning there might be some traffic on the Grande Raccordo Anulare beltway). I know it's hard after a long airplane journey from down under. But 3 trains (with luggage) and a bus ride for a total of at least 5 + hours are not ideal either.
PS: an alternate drive to avoid the GRA (Grande Raccordo Anulare beltway) is to take the coastal route using the A12/E80 freeway north from near the airport and drive via Civitavecchia-Grosseto-Siena (40km shorter but about the same length of time).