It's worth reading a bit about the history of BART. The Bay Area's NIMBY roots run deep. Initial plans would have included lines in the peninsula, with eventual connections all the way around the bay. Santa Clara County never joined, and that started a cascade of other counties dropping out which financially crippled the project and left only the scaled-back system in place today.

https://en.wikipedia.org/wiki/Bay_Area_Rapid_Transit#Origins...