Bob2 had a point in his post that many of us miss: the "commuter" air corridors between N and S Calif have so much service that overnight trains are just not that attractive. There are many flights a day per airline, multiplied by the number of airlines, such that you can probably find a (1-2 hours, with probably another hour added for airport shenanigans) flight near when you want to go or arrive, and no need for an overnight stay unless you have a very early meeting in the morning.

Present-day conventional Amtrak trains can't compete with that over a 400-600 mile run, though they do a good job in the shorter corridors - does anybody actually fly from LA to San Diego when they can ride a frequent Surfliner in 3ish hours? OTOH, with the fluid situation in office employment right now, I could see trains becoming more competitive if: 1) there were more departures/arrivals in suburban areas (home office users), which airlines just can't do; 2) there were more runs overall; and 3) higher speeds were possible on main lines (40-60 mph trains, as on LOSSAN north of LA, just don't cut it any more, and HSR or not there needs to be a quicker way through the Tehachapis).

Finally, how are the overnight buses doing? Yes, there's a redeye bus connecting the Cap Corridor and Surfliner; what's the ridership? Do they need extra sections? How's that Santa Monica bunch doing? Really, I see overnight trains in the greater California Corridor (Sacramento-San Diego) as very much a niche thing that would need a lot of marketing and something distinctive (not just another seat) to draw traffic away from daytime commuter flights and driving. And if you're going so far as to run to Sacramento (did SP actually ever do that with a LA train?), why not Redding? Or even Reno? Spitballing is fun...