SMART/NCRA's track ends at the Brazos Bridge or just east of there. Cal Northern emphatically isn't interested in passengers. So forget Vallejo and Napa and Suisun unless the traffic is there (hint - probably not, at least as demonstrated by the minimal bus traffic on Greyhound) to justify building a new railroad.

GGT doesn't serve Sonoma even though it's in their regional territory. Some years ago, they did have a few commute runs from there into SF, but they're long gone as are the Sebastopol commute runs. Unknown how many "club buses" come from those areas, but it's unlikely to be more than a few (the E. Petaluma and Rohnert Park club buses when I lived there kept it to 4-6 runs for feasibility of volunteer management and minimizing competition with GGT's routes). Therefore, little demonstrable traffic potential for commuter train into Sonoma.

IOW, until the roads congeal entirely (yes, I know 37 is getting there, but has been for many years and drivers have adapted), a non-road-based transit alternative in the North Bay outside of the 101 corridor is simply not needed.