How tight?

With each tunnel track directly connecting to one island (pair of platform tracks), clearing a pair of platform tracks then filling them would be an even longer period the platform is idle.

From the arrangement described in your diary, at least the Caltrain platforms can be reached from two of the tunnel tracks. Making just one of the tunnel tracks bi-directional, e.g. the Caltrain departure track, would already be an improvement. It would add the possibility of a Caltrain and a CHSR train arriving in parallel, which, due to eventual delays, is more important than the possibility of two departures in parallel.

E.g., in addition to these possibilities:

...now you would gain this one:

Even better would be another connection between the two outer tracks, so that a Caltrain departure opposite a parallel Caltrain+CHSR arrival is possible.

_{*Lunatic*, n.
One whose delusions are out of fashion.}