Traveling second class by train, it would cost you 12.34 eur each way, per person, for a total trip time of 3 hours 11 minutes, as follows:

Departing Roma Termini (#2312) at 13:14, arriving at Chiusi-Chianciano Terme at 15:01, changing trains and departing that station at 15:09 (#6916), Arriving at Siena at 16:25.

The info above is from this web site: http://www.ferrovie.it/fit/index.php Roma / Siena arbitrarily using: 29 / Lug / 2005 13:00

Hope that helps, Linda