We stayed in a hotel just past the train station. The walk is about 15-20 minutes to the square. It's another 20 minutes or so to the entrance of the museums.
The number 64 bus begins its run just outside the train station, and will stop right across from St. Peter's Square. There will usually be one waiting there when you get off the train; most of the day, they run every ten to fifteen minutes. You can't miss the stop, unless it's early in the morning. The bus will be practically empty when you get on, but at St. Peter's Square, there will be a crowd waiting to board.
If you buy a single-fare ticket(€1.50), you can use it for both the train and the bus.
The walk is mostly downhill towards the Vatican, but uphill on the way back.
Bvlenci missed the bit about how you'll be traveling from the Trastevere train station, not Termini.
From the Trastevere train station, you'll want either the FL3 or FL5 towards Cesano. The San Pietro stop is the second stop. It's about a 10 minute walk from there to the Piazza.
No, I knew it was from Trastevere. When you get off the train at San Pietro station, the 64 bus begins its route there, heading to Termini. It has very few passengers between San Pietro and St. Peter's Square. I stayed in that area several times when my daughter was at the American Academy.
I think the walk is more than ten minutes, and even more on the way back. Since the same ticket is good for both train and bus, I don't see any reason to walk. The route isn't charming.Edited: 3:21 pm, January 06, 2016
My apologies, bvlenci. I misread your response. My 10 minute walk estimate was based on Google Maps, but Streetview does indeed look as if it would take longer.Edited: 3:37 pm, January 06, 2016
I did the walk, actually from further away than the station, in the summer, and we weren't walking very fast.
-:- Message from TripAdvisor staff -:-
This topic has been closed to new posts due to inactivity. We hope you'll join the conversation by posting to an open topic or starting a new one.
To review the TripAdvisor Forums Posting Guidelines, please follow this link: http://www.tripadvisor.com/pages/forums_posting_guidelines.html
We remove posts that do not follow our posting guidelines, and we reserve the right to remove any post for any reason.