The DTX approach tunnel to the Transbay Terminal station throat is a horrible design. Its supposed to be three tracks wide, but with no better capacity than a well designed two track tunnel would have, due to an insistence on placing the intermediate underground platform terminals directly on the outside two lines, so the entire length of the central track through the entire tunnel is occupied with the task of juggling Express trains past the platforms.

With two right angle turns in the approach, and the curve radius for three tracks in a shallow cut and cover tunnel along the street grid, its obviously not a high speed tunnel. I'd expect 5 minutes minimum lost to the traverse between the tunnel mouth and the TBT station throat, leaving the Caltrain run to be 25mins or under to achieve a SF/SJ time of 0:30.

OTOH, they just started getting serious about value engineering at Parsons-Brinkerhoff when it looked like the $100b Year of Expenditure price tag was going to knock the project out due to sticker shock. If the Governor's office puts ongoing pressure on keeping the costs down and performance up, they could still change to siding platform track with high speed switches at the underground 4th and Townsend station and a two track tunnel, which would allow for higher radius curves in the DTX, simpler operation, and pick up a minute or two at a saving of money.

I've been accused of being a Marxist, yet while Harpo's my favourite, it's Groucho I'm always quoting. Odd, that.