A lot. It's also not a good use of money, as BART has an unusual track gauge, strange engineering decisions, and is operationally and mechanically incompatible

A lot. It's also not a good use of money, as BART has an unusual track gauge, strange engineering decisions, and is operationally and mechanically incompatible with other commonly deployed rail systems. Not to mention, even with BART fulfilling a strange role between an in-city rapid transit and far-exurb commuter rail, extending lines further out from the operating core comes with issues around trainset supply and capacity allocations in congested areas, like the Oakland Wye, the Mission Street Subway, or the Transbay Tube. It's far more likely to "extend" fringes of the system with other modes that can then operate independently, like the eBART [1], ACE, or the Capitol Corridor, or even a bus line. Extending the BART entirely around the western side of the bay won't happen, disproportionately due to the city of Atherton [2]. It's also silly, considering Caltrain fulfills a very similar role in transit to what BART would bring to the table, along the identical corridor. I once wrote [3][4] about why the geography of the current transit options in the Bay are unfortunate, and single mode of transit completing the entire circle would do little to alleviate the problem that so much of transit around the Bay is already around this circle. [1] https://en.wikipedia.org/wiki/eBART https://en.wikipedia.org/wiki/eBART [2] http://sfist.com/2016/07/11/atherton_wealthy_will_still_maybe_t.php http://sfist.com/2016/07/11/atherton_wealthy_will_still_mayb... [3] https://news.ycombinator.com/item?id=12282756#12283335 https://news.ycombinator.com/item?id=12282756#12283335 [4] https://news.ycombinator.com/item?id=14277421#14278045 https://news.ycombinator.com/item?id=14277421#14278045