Is it possible to connect to a high speed train going from Siena to Rome?
Pamela,
You would have to go to Florence to catch a high speed train and that wouldn't save you time.
The Sena bus takes just over 3 hours from Siena to Rome the train can be a little faster or a little slower depending on connections. For the train see http://www.ferroviedellostato.it/homepage_en.html