[sliverstorm]

BART isn't really "underground" though, right? I was pretty sure for 99% of its track it never goes deeper into the ground than an underpass.

[potatolicious]

Outside of SF yeah, a lot of BART is elevated or separated at-grade, but in the city proper it's definitely legitimately underground.

Which is the same as NYC - subways in the central bits, elevated/at-grade rail as you get further out and land less scarce.