The San Joaquin Valley Rail Committee has looked at this for a while and even written up proposed schedules. Aside from the UP's longstanding objection to a route through there, I'm not convinced that it would do well. All of these schedules pass through at night, and basically most passengers would be trying to sleep in a California Car that barely reclines, with those bright lights and announcements every hour every time the train hits a stop. You'd have to get a sleeper somewhere to use on this train, and that would only carry a few dozen people.
While there may seem to be a lot of people on the overnight bus from LA to Bakersfield, in reality that is very peaky and on weekday mornings it might have a couple of dozen people at most. The northbound bus-train combo, while leaving LA at almost 2 AM, actually seems to work for my trips up north, but now I generally am considering Megabus for the return if I don't feel like flying or if this is a spur of the moment trip with $150+ air fares. I don't think being held hostage on a train in the middle of the night, with "recovery time" stops in Palmdale and Bakersfield to watch the clock catch up due to all that single track, is that great an idea. The Amtrak detour concept actually works pretty well. Run several trains a year to allow foamers, railfans, etc. to ride over Tehachapi and enjoy the scenery. The rest of the year, the bus is not ideal, but it's a heck of a lot more cost effective than sending a few dozen people on an eight hour ride over the mountain, just so they don't have to get up.
