> The idea of using existing connections could work, but it
> would make more sense if BART or Caltrans could ensure connections
> with short wait time.
I'm sure that could get worked out. But the real issue is very few of the projected passengers would be going to an area that's close to downtown San Francisco, anyway. San Francisco is not Paris[1]: the vast majority of people in the Bay Area live in the location called "not San Francisco" -- so if you arrive in downtown San Francisco, chances are very good you have to connect to some other transit anyway. Why not transfer early, to an integrated grid, at the periphery?
We're not talking about going to Hoboken because you can't afford to build the final 2 miles of track to penn Station; Livermore is 40+ miles from San Francisco; San Jose is 50 miles. Go 50 miles from Paris and you're way outside Ile-de-France, in the sticks.
/jordan
[1] The Department of Paris (75) is about 40 square miles and contains 2.1M people; the City of San Francisco is about 50 square miles and contains less than 800,000 people. Paris and the "inner ring" is about 300 square miles and contains 6.5M people; the Bay Area is commonly referenced as ~7000 square miles with a total of slightly more than 7M people. Paris is one of the most densely populated cities in the world: it is the largest-densest city outside of India (which has four cities that are larger and more dense: Delhi, Kolkata, Chennai, and Mumbai). San Francisco is nothing like it. San Francisco isn't even the largest city in the Bay Area: San Jose is.