The problem is not a lower deck for trains on the Golden Gate but the cost of the approaches at both ends. The Waldo Grade from Marin City (sea level) to the height of the bridge would require a very circuitous grade. On the SF end, neither Doyle Drive nor Park Presido could accomodate the trains. A subway to the downtown area is the only viable option. Both engineering challenges at the North and South ends are solvable BUT only at enormous cost. The population and tax base of the North Bay do not justify such an expense. SMART to Larkspur (and ferry to SF) is the best fit for the region.