BART exists. This isn't different. Put a couple tunnels under the bay, one just north of the bay bridge and the other at SFO. Connect them, staying safely east

BART exists. This isn't different. Put a couple tunnels under the bay, one just north of the bay bridge and the other at SFO. Connect them, staying safely east of the San Andreas fault. Try to route the tunnel to allow stopping at at least one of the other airports in the area. No, it isn't cheap.