It was impossible to build the golden gate bridge until engineers could figure out how to transport a lot of people a far distance while using a really small earth footprint. To build a transportation system across the city we don't have that demanding requirement, but by giving ourselves that goal we might get some good ideas.
Edit Nov 26: I keep thinking about this idea: Here is an image of how this could be realistically implemented. Think how much lighter and cheaper you can make a bridge when you remove the entire weight of the road. Is this simpler and more elegant than digging a subway?-------------------
Edit Dec 04: So I just read that San Francisco has a proposal in the works to extend their subway. There's a heated debate. It's a much needed addition, but it's $1.2 billion. Here is the proposed route. 1.2 billion $? This idea is really starting to seem important now. Yesterday, I biked the route and thought about the logistics of building a system like this. Here's a sketch of a SkyRail station above the existing CalTrain station.
I need to get a faster computer to render videos like this, but here is a really crappy version of the first part of the route.
I'm talking to a structural engineer to make sure this concept is feasible, and then I'll render a video of the route and photoshop some stations to make this proposal seem really possible.
I think this is worth thinking about as an alternative to building a subway. It could be an beautiful iconic structure. The cable car was a ridiculous system too when that was proposed. Think about it, a huge loop of cable miles long that a car would grab on to? Weird.