It's a pity that the United States didn't embark on a concerted effort to build a high speed rail system back in the 1950s and 1960s, when we had more money and a smaller population. And fewer lawyers. Instead we wasted a couple trillion (adjusted for inflation) on Vietnam. Think what we could have done with that money not to mention thousands of lives saved.
It seems to me that California's highway problem is one of simple over crowding. Even if they built ten high speed rails between SF and LA, it would probably not make much of a difference, and I'm not even talking about the issue of needing a vehicle once you arrive at LA central terminus. Trains carry hundreds of people; let us say, a few thousand at any given time might be carried on high speed rail cars, but this is a tiny fraction of the millions of motorists clogging the highways 24x7 all up and down the coast.
I see no solution to California's clogged highways other than to siphon some of the population off to neighboring states that are vastly less populous. Perhaps this will happen naturally as technology makes it easier to disperse workers over broader geographical areas. It seems to me that huge cities are a legacy thing anyway.
Probably the next big thing will not be speeded up 18th century iron horses, but electric long range buses and self-driving cars that will safely and quickly negotiate the computerized traffic grids of the future.
California ought to spend a few billion on embedded markers and sensors in its roads that self-driving vehicles can access; probably this will be a much better investment in the long run.