As far as bi-directional track, its a mile plus of tunnel with two very tight turns, and transit time to clear the track is likely to be fairly long. 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.

With the system pressed for platform dwell time, the freedom of completely parallel movements for each island comes at a substantial reduction in total train capacity.

Now, without a fracking through track looping back to the present terminal (clenches fist and shakes it in a vaguely western direction at the TBT authority), dedicated two-way track has substantial higher throughput capacity than bi-directional track.

The penalty, of course, is that there is no longer completely parallel movements per island.

However, with a central access track and an egress track on each side, then it can be set up to allow parallel HSR access and Caltrain egress, and parallel Caltrain access and HSR egress.

That means that if Caltrain and HSR services enter in alternation, there is an open slot for HSR egress before each HSR access, and an open slot for Caltrain egress before each HSR access.

It could equally well be two access and a central egress, but since the total station dwell capacity limit is at the TBT trainbox side of the tunnel, if the access and egress capacities are to be unbalanced, the larger capacity should be allocated to egress.

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