Actually, SP reopened the Tehachapi line in 25 days according to news items of the time (1952) still available or reprinted in later retrospective articles online. Of course, it took them longer to do permanent (as permanent as anything is in CA) repairs, including daylighting at least one tunnel.

With HSR using mostly tunnels & bridges in the mountains (little space for actual at-grade construction as the current line uses), there's potential for a quake taking longer to fix. Hard to do a shoofly of a long bridge or a mile-long tunnel. But as discussed elsewhere, that's a fact of life when engineering a major transportation line in California's active geology. You design as well as you know how, and if possible allow a bit extra for the inevitable problems.