Subways are typically speaking not he most economical form of mass transit.  Right of way, interruption and building underground tunnels (not open trench) are super expensive.<p>That said an above ground system up and down over and across Van Ness, Geary and maybe Judah&#x2F;Taraval and taking over for Caltrain down the peninsula would be great.<p>Speaking of the peninsula, it could exploit a system going down the camino real corridor.  That alone could alleviate congestion and housing, if only they developed that long corridor and put in mass transit along the line and allow mixed use mid-rises five blocks on either side of it.