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)

I’ve read that it was San Mateo county pulling out that made it economically unviable to include Marin (as well as disagreements between BART and the GG bridge district about the engineering viability of going across the bridge).

That's right http://www.marinij.com/article/ZZ/20100807/NEWS/100809186