It's a good idea on paper to tunnel underground and have a stop near the proposed Warriors Arena, but now some serious opposition to the basketball stadium is starting to surface that could keep it tied up in litigation and prevent it from ever being built. There was a separate plan to just tear down the 280 freeway extension over the Caltrain tracks and build a surface roadway like the one on the Embarcadero but I don't see how that could be done without shutting down Caltrain for months.
A rough guess. It would probably be at least 10 years before they could start construction on the Mayor's proposed rail line The article doesn't state how the rerouted line would connect to the new Transbay terminal