Power is only part of it. Presumably the wiring is done for compatibility without major (if any) upgrades, because that's the expensive part. Adding additional capacity at substations is relatively minor work if everything in the ground and on the poles is ready for it.

The real problem is speed and grade crossings. Despite an ongoing grade sep program, Caltrain still has a lot of them, and to safely run at even 110 HSR will need to fund quite a few more. So while Caltrain is pretty much ready, it's certainly not at 100%. On the plus side, it's going to be a long time before CAHSR arrives (if it ever does), and we'll get a lot of good use out of the electrification before that happens while Caltrain and the region continue to nibble away at the grade sep needs.