This idea is completely nutty.  It seems like the Mayor’s office is so wrapped up with the idea of tearing down I-280 and adding to Mission Bay that they haven’t thought about how to actually build a railway tunnel.

With this design, Caltrain and High Speed Rail will need to make THREE right angle turns, two very close to each other under 22nd Street (right next to the existing station?) as it approaches the new station at the Transbay Terminal. Also, the storage capacity at Fourth and King (needed to run frequent service particularly at rush hour) would be lost.  And though it would be interesting to have a Caltrain/HSR station by the arena, there already IS one at 22nd Street – a station that can be improved at vastly lower cost than this design.  

And of course the arena site is ALREADY served by light rail, which will be better once the NIMBYs blocking the Dogpatch turnaround are finally stopped.

This is not to say that the current proposal for tracks to Transbay is ideal (it too has right angle turns) but this would be far worse.

Fortunately it appears that Caltrain at least is not taking this seriously!