[a software traslated this text... sorry for mistakes. Original italian text here
The connection from Rome to Siena is one of the most simple and comfortable throughout the transport network departing from the capital.
I did back and forth for two years because of the University and I have always found it very well.
Train would required a change at Chiusi and then take a bus to reach the center of Siena - the bus is to be comfortable and always punctual.
One day trip: Rome to Siena (by bus)