Fun fact: the reason it's like that is because both levels were envisioned for BART usage before the Peninsula lines got cut. In the original design both levels would have been the same fare area and you would have been able to walk between them instead of having to take the big escalators down to BART caged off from the Muni level. It's comical to watch one of the Muni trains crawl to one end of the giant platform that was sized for 10-car BART trains.
Ah, I've always wondered why the Muni platforms (especially Powell) are so ridiculously long compared to the Muni trains themselves. Makes so much sense that they were originally designed for BART cars.
“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)
