Earlier studies did include tunnels through the mountains roughly paralleling I-5. If I recall correctly the run from Bakersfield would have had to climb a long fill to a few 100' above the valley floor (crossing a very active earthquake zone - interesting), then enter a steep (more than 2% grade) tunnel that daylighted around Gorman someplace, crossed the S.A. Fault, then dropped into a matching tunnel down into Santa Clarita. Ungodly expense, probably impossible to run at any speed without maglev (which was considered at the time), no on-line business to help cover it. Going over Tehachapi is a little longer, but far more practical from an engineering standpoint and has the advantage of on-line business in the Antelope Valley. There's a reason why SP went that way, and the HSR people eventually listen to people who are not dummies if they're hit hard enough often enough.

Scratching the political itches of N. LA Co. didn't hurt any either.