BART is actually a heavy rail metro like the Tube, not a light rail system like Muni's Metro rail or the San Jose system. While I'm no huge fan of it (there are numerous flaws, among them being that it's non-standard in just about every way it possibly could be), but one remarkable thing about it is how huge the system's reach is. Have a look at Subway Systems at the same scale; BART actually covers more territory at its edges than the London Underground, although of course it only covers a tiny bit of that area, and connectivity is awful, given that there's something like 23 separate Bay Area transit agencies, every one of them engaging in "Your children must die so that mine may thrive" politics.