The Golden Gate Bridge was never designed for a 2nd deck or rails. A close look and photos during construction show X braces under the roadway clearly indicating that a 2nd deck was not considered. There are also major geographical barriers to a northern rail crossing of the Bay.

First, any railline would have to go from sea level at Richardson Bay to several hundred feet to reach the north end of the GG. This is now called the Waldo Grade. Then it was a U.S. Military Reservation. Today its the Golden Gate Nat'l Recreation Area. No way anyone is going to allow construction of a railorad ROW through such a sensitive area.

Then there's the south end. You have to get the trains down from the bridge level to sea level. In addition, the GG bridge is miles from downtown SF. Trains would need a subway through some of the densest (and most expensive) residential areas of the City.

Back in the late 50s or early 60s when BART planning was just beginning, the only possibly viable plan was a crossing from San Rafael to Richmond by bridge or tube to connect with the end of the BART line at Richmond. The expense of such a crossing coupled with the more limited population base of Marin made this plan unworkable.

Its important to keep in mind when discussing SMART that they are not really targeting commuters to SF. SMARTs analysis shows most potential riders traveling from their homes in Sonoma Co to jobs in Marin.