- The basic geography of the Bay Area: it's big, it's polycentric, it's hilly, there's a huge body of water on the direct route between many origin/destination pairs. The train ride can take hours and can actually cost quite a bit.

- America's inability to build mass transit cost-effectively. At this point train rides along BART are mostly between high-cost areas, and discussions of expanding the system even a little more have price tags in the tens of billions, and timelines of a decade or more. Besides, the radius of BART is about as wide as it ought to be — it should be filling in, not sprawling out, and that requires upzoning the areas along the train to be effective.