The original BART map shows the Marin line branching off around Montgomery Station. It was later that engineers realized the difficulties in tunneling through the Chinatown and North Beach areas that focus shifted to a Geary St. extension. Before any serious survey of that route could be completed Marin withdrew from the system and the idea of a Geary extension, now a stub end, was abandoned.

A Geary Corridor line has been a dream for Muni for years. It would probably be an extension of the current Muni Metro rather than BART. For those unfamiliar with the topography of San Francisco, running BART from the Market St alignment west along Geary would require surmounting several hills (Remember little cable cars climbing halfway to the stars?)

Another very real obstacle is that San Franciscans living near the western end of such a line fear hoards of Marin commuters trying to park in their neighborhoods to ride the Muni to jobs downtown. A fear which I believe is justified.