If I had a job then like I have now, I would have used that overnighter. One serious issue is in fact lack of late-departure/early-arrival service between LA and SF or Sac. Last bus out of LA for the San Joaquin is at about 3 PM (arriving Sac about 11:30P - Oakland after midnite by bus) - doesn't work if meetings run until 5 like they often do. So you catch one of the last couple of flights out of BUR or LAX, hoping they'll be somewhat on time, and get home late evening expected to be at work the next morning and performing poorly on 4-5 hours sleep. This is where something on the Tehachapi line might work: since speed isn't really the issue, you could take 3-4 hours to get from LA to Bakersfield. If running time from there is approximately normal, it would result in a 9-10 hour trip - say 9 PM to 7 AM - that allows a full day of work on each end and some rest (even in a coach seat) in between. Call it the California Red-Eye. With "normal" SJs taking (including 3 hours for the bus) 8 (from Sac)-9 (from Oakland) hours for the trip, a 9-10 hour trip with train all the way (OK, probably bus btw Stockton & Sac with a 6 AM transfer) should be doable. Skip all but the big stops & call it a Z train instead of an Amtrak for dispatching purposes.

The old attempt at an overnight via the coast suffered not only from lack of marketing, but also near-zero online business (not many people live in those coast towns who might be willing to catch at train at 0-dark-30; s few in the Valley might) and too long a run (14+ hours via the coast from Sac to LA). An overnight resembling the old (Owl??) through the SJValley would have better departure/arrival timing.

Problems include no equipment to do it with (assuming ability to get slots from the RRs). Nobody would want Cal Cars to be stuck all day in LA waiting for the next night's train back (equipment has to be used better than that). Cal Car seats aren't really very comfortable for sleeping in anyway - would need Superliners coaches. And Amtrak really doesn't have any spare sleepers. And given the grades on Tehachapi and the probably longer train you'd need 2 locomotives instead of 1 (for protection too).

Anyway - worth thinking about and theoretically doable even without any increased train speed. Yes, I know there's a night Amtrak bus on the coast, and a SJ bus leaves LA at about 2 AM (too early!), but their connecting trains get to Sac/Oak too late for a full work day and require a bus/train transfer in the wee hours. With some marketing, I think an overnight train via Tehachapi and SJValley might work.