I think Brandon is right. I do think that the 4th Street Subway is the solution that is in place for handling the transport of our residents and visitors to locations North and South here in San Francisco. It will be a tremendous asset to the city and surrounding areas.
Here are a couple links for your reference:
I hope this is helpful.
Not at the moment, no. There is a plan in place (and preliminary construction has begun) to run a subway from the Caltrain station at 3rd and King to Chinatown. I think that eventually the hope is to continue this line all the way down into Fisherman's Wharf and then possibly through the Ft. Mason tunnel and along the Marina Green, but that's only speculation at this point.
Oringainlly BART was to run to Marin and San Mateo but they opted out. If it had gone to Marin it would have been built under Geary and SF would have been better served. As it is now only suburbs are seeing any extensions, and the airport.