“The San Francisco Downtown element of the Bay Area regional rapid transit system consists of a four-track, two-level subway beneath Market Street and a two-track, single-level subway beneath Post Street.”

“At Montgomery Street, the Market Street subway joins the San Francisco approach to the Trans-Bay Tube. The subway extends up Market Street to about Van Ness Avenue where it swings lo the south to become the Peninsula Line in Mission Street. The lower level of the subway provides through regional service by joining the Peninsula and the Trans-Bay Lines. The upper level is built to accommodate local rapid transit trains at a future time and will be utilized initially by the streetcars of the San Francisco Municipal Railway.” (emphasis mine)

And the flow map of estimated 1975 passenger counts makes it clear why they would want to double up on platform capacity along that stretch: https://www.bart.gov/sites/default/files/docs/50-years/1961-...

Imagine if we got this BART: https://i.imgur.com/hon9nEf.jpeg (1956)