Actually, "fantasy" is not the best way to describe the subway exercise, though it is entirely fanciful. First, it's a political ploy by Scott Wiener in his campaign for the state senate: he's a man of vision! And it's a PR stunt by City Hall deployed by some of the MTA's many employees.
From the MTA's blog:
To ensure our city remains vibrant and livable, under the direction of Mayor Ed Lee, the SFMTA, the Planning Department and the San Francisco County Transportation Authority have been working hard to make sure our transportation system keeps pace. The Subway Vision is just one piece of a new effort to create a broad, long-range plan for all aspects of transportation called Connect SF, which is meant to lay the foundation for an effective, equitable and sustainable transportation network for the city’s future.
Though fantasizing about trains isn't as cool as the bike fantasy, Streetsblog likes this campaign because trains aren't cars, which is why it supports another fantasy: the high-speed rail project.
Curbed likes the campaign but injects vulgar reality into the discussion:
Of course, the awkward subject of precisely how we’d pay for all of this inevitably came to rain on the collective parade eventually. The final proposal will probably be much less ambitious than everything talked about now, but why not let the people dream?
Yes, there will apparently be a final subway "proposal" by the city later this year, which will presumably have some numbers about what it will cost: $1 billion a mile to dig a subway anywhere in the city.
The hed on SocketSite's story: Here’s Where San Franciscans Most Want New Subway Lines. In fact only 2,600 people participated in the subway PR stunt---in a city of 866,583 people.
Your tax dollars at work: