Another train line would be a waste -- take the window seat on Caltrain and look how utterly empty it is the second you leave SF. The whole way down the peninsula the train is flanked by flat, low-density construction spotted with breaks of wide open space. You don't see any real high-density housing until you get to San Jose.
Like LA, it seems like there needs to be another train line. For us it's Hollywood/Beverly Hills. For the Bay, it'd be, maybe Sunset down the 101.
But maybe I'm wrong, and two options just mean you're more likely to be incorrect on either side?