The good folks in the Richmond District wouldn't want the BART people either - and there's really not enough on-line business to justify regional commuter heavy transit there. If you're going to have a 2nd BART line in SF (complete by some time in the next century?) the detour via GG Park looks good - except that you would have to tear up the park for the tunnel as well as 19th Ave which the Sunset folks would let you know about. I seem to recall a freeway revolt precipitated in part by the prospect of tunneling in GG Park...

As noted by some above, I wonder if the congestion is caused by the tube itself or by the station situation on Market St. Unfortunately, fixing either would be a multi-10s of billion$. You can't easily go under the existing BART stations because Muni is in the way at 3rd Street with its super-subway. And of course the places in the U.S. let alone the Bay Area where you could build tube sections don't exist any more - you would have to ship them from China just like the Bay Bridge pieces.

Wasn't the tube originally supposed to have trains every 90 seconds at rush hour? That's not happening right now.