Transportation from Termini to St. Peters

Metropolitana (subway) line A. Get off at Ottaviano station.
If you travel with lots of luggage, it's better to take a taxi. It shouldn't cost more than €15.

Are you coming from the airport? If so the easiest would be to take the local train from the airport to St. Peters stop and a very short , if not walkable, cab from there.

I plan on taking the Express train from Fiumicino to Termini. Is that the smartest choice? I read take the train then walk... is there a better train choice that I am not aware of? I only travel with a carry on luggage and small day pack.

No, it is not the smartest choice if you want to go to the Vatican. The Leonard Express is the 1st class train that is non-stop from the airport to the train station, Termini. It is 14 Euro. Then you will have to take a taxi or bus or something back across Rome to the area of St. Peter's Square. Probably another ten Euro or more.
But there is a local train leaving from the same station as the L Express and it makes two stops on the way to Termini. One of the stops is San Pietro station which is quarter of mile or so south of St. Peter's Square. From there you could probably walk to your convent but I don't know exact where it is. Just use one of the map apps to figure the distance. The fare for the local train is 8 Euro, as I remember. And the train is FM10. That could be wrong. Just ask the ticket agent at the airport station where you board the train. Cheaper all round and save a long taxi ride across Rome. You cannot miss since it is the only local train from the airport.

Wonderful, I will look into the local train. I am female 62 and I am very excited about my adventure to Italy but the logistics has me wondering why I am doing this. My only other logistical nightmare will be getting from Rome to Positano (with a visit to Pompeii). Thanks for the info, I did not know that there would be a local train from the airport.

We just returned from Rome. We arranged for an airport shuttle to pick us up from FCO to our guesthouse next to St. Peter's, just so we didn't have to lug our bags on and off trains. I booked it at www.airportshuttles.it for 26 Euros. Expecting a small bus since we did not book private transport, we were pleasantly surprised to be picked up by a town car since we were the only ones arriving on that day. Easy trip, enjoyable conversation with the driver.

Just an option you may want to consider.
To compare, we did take the Leonardo Express from Termini to FCO for our return flight - our experience was just OK. The train was so slow, it arrived later than scheduled at FCO, causing us to miss our hotel shuttle (another story !) The train car smelled of urine, the only time during our travels in Italy by train that we had an unpleasant experience. And for two travellers, it cost 28 Euros, 2 more than the airport shuttle.

I usually take the local train into Rome from FCO. I get off at Roma Ostiense and then use the Metro which gets me to where I need to go. I don't remember the train stopping at St. Peter's, but there may be another train besides the Leonardo Express and the local that goes to Sabina/Orte.

Correct, you have to change trains at Trastavere to get to San Pietro. It still beats taking the more expensive Leonardo Express to Termini and then changing to the Metro.

After do-it-yourself traveling in Italy for 34 years, this year we used romashuttle.com and wondered why we didn't use it before. We were met at the airport and taken directly to the b&b across the street from the Roma Opera House. No muss-no fuss. The cost was 45 euro and was worth every euro.

Buon viaggio,

You can get train schedules from Roma Fiumincino to Roma S. Pietro either at the airport station or print some from DB before you go.