I don't believe there is enough potential commute ridership to make SMART viable. SMART should be the trunk service from San Francisco to the North Bay. A joint operation with San Francisco Muni would be a win-win for both systems. I think the best argument could be made for access via a subway along the Geary corridor, as this route already has the patronage that cries out for rail service. For Muni the regional link would bolster the case for funding with MTC and the Feds.

I am sure that BART has a scheme along these lines stashed away somewhere, but BART is not the desirable mode imho. It has capacity overkill which would rightly scare off environmentalists and requires absolute grade separation, not to mention the gauge problem. Standard electric light rail service offers the possibility of being extended into Mendocino County.

Once the freight railroad issue is resolved once and for all the whole idea of North Bay mass transit won't seem so radical and over the top. No version of SMART can succeed without some serious infrastructure spending. An extension to Marin City, for example, would be expensive, but consider the vast and fabulous sums BART is prepared to spend on its San Jose line. These projects have to be viewed as long term capital investments - like the NYC subway system - been here 100 years and will be here in another 100.