Siena's tourism service office is useless in this.
That's their reply for my question:
"all the info that you wrote down sound correct to me according to the info given by www.griforama.it .
Please double check at the train station when buying the tickets."
I rented a car and drove to Monteciano, because Rick's book mentioned it. You should drive over Monteciano for 4 km. I parked my car at Monteciano under a sign for "parking San Galgano", and following the walking trail sign, which claims to be 2km. But the sign must be years old, as a bridge over the river was taken off.
For public transportation, this is the closest of what I can find, as I asked the Siena tourism service office:
" Could you please tell me know to go to Abbey San Galgano from Siena? Using public transportation.
Here're the two options I found. Are they still correct?
1) bus 122
leaves the train station at 13:40 goes to Palazzetto. cannot find a useful return schedule.
2) the Siena-Massa Marittima bus.
leaves the train station at 14:05 goes to Palazzetto. returns from Palazzetto at 17:25.
Is Palazzetto the correct stop to get off the bus?
How far is Palazzetto from the Abbey? 30 minutes' walk?"