City Insider points out a "concept" to build a transit line in SoMa along either Folsom or Townsend. The idea comes from a report authored by urban think tank SPUR addressing the issue of adding more jobs to the downtown "core."
The obstacles at the moment, as they've outlined, are myriad: lack of space for more offices, the city's goal of "sculpting" a particular skyline shape, a cap on the amount of new office space, and condos and retail crowding in on office uses.
Based on the principle that people are more likely to take transit to work based their transit stop's proximity to work, not home, one SPUR idea is to expand office use beyond the Financial District to SoMa, and to build a transit corridor along either Folsom or Townsend.
Other ideas: more offices in Mid-Market, 4th Street, and Market/Van Ness; expansion of Central Subway to the Presidio; and bus rapid transit on 16th, as well as Folsom and 7th. Fantasy transit's so fun.