You can't visit Pienza and Montepulciano by train from San Quirico. There is no train service in any of these towns, but there is bus service.

I repeat: If this is now a cycling trip to particular towns, at least north of Rome, you need to rethink the route and timeframe. And add, if you want to combine cycling with public transport here and there, you need to research what's possible/realistic in the time you have.