Here's a good article on it from 2010 discussing the several reasons for BART not building to Marin:

[www.marinij.com]

Sounds like there was tax base, bridge problems, voting issues and more...ointeresting history.

Yes, no evil conspiracy except the Bridge District appearrs to have engineer-shopped to support it's desire to not see BART cutting into it's toll revenues. They found an engineer to shoot down the idea of rail across the bridge.