I'm sorry to disagree Mr. Mouse but the GG bridge was never planned or designed for double decking. It was not until the original concrete roadway was replaced that this became even remotely possible.

There are several problems relating to commute trains on the GG not all of them engineering ones. First, how do you get from sea level at Marin City to the height of the roadway at the north anchorage? Have you ever driven over the Waldo grade? It would require several miles of loops or switchbacks or whatever to accomplish that. And through a national park at that. Then what to you do at the toll plaza? Its miles from SF's downtown and any rail line would have to go through, under or over some of the most expensive real estate in The City. Out Park Presidio to Geary and a hard left to Civic Center? Again, try driving that route and note the elevation changes.

OK, the above problems could be solved, all it would take is $$$ and by the bushel. BART says 300,000+ use the trans bay tube daily. The GG bridge district claims less than 100,000. The potential traffic just isn't there to justify the expense. The total combined population of Marin and Sonoma counties is less than Alameda County alone.

Finally, the voters of SF would never approve a tax hike to tear up large portions of their fair city to benefit the commuters of the north. Ain't gonna happen.