Rail Service (SMART) begins today between the STS station (not right on site but close, with a bus link) and San Rafael. The frequency and hours aren't great, but it is another option for those in the area.

http://www.sonomamarintrain.org/stations

Eventually, the trains should go as far north as Cloverdale, and as far south as Larkspur (where one can catch a ferry to SF).