A subway under Geary downtown and through the Western Addition, surfacing at Steiner Street, was proposed as early as 1936. It would have used conventional streetcars, and, had it been built -- at a then-projected cost of $13 million -- it might have forestalled the automobile expressway.How much would a similar subway cost today? In the billions of dollars. It's needed but expensive. If we don't do it sooner, it will have to be done later, and at an even greater expense. Such is the cost of waiting. If we had done it before, the tunnel would be there for good and we wouldn't have to lay down billions.
I also suggest reading Transbay Blog's excellent write up.