In the past, I heard the story that BART was blocked in Marin by wealthy NIMBY's who didn't want to share, but just recently I learned this was BS. Turns out BART canceled the Marin expansion because: 1) GG Bridge couldn't take the extra weight of another deck; 2) Building to Marin would have make the whole BART project too expensive at the time, so Marin got cut. Interesting! (Thanks to Bay Curious)