This is an interesting idea - essentially, it sounds like the plan would have Metrolink operate the train LAX-OSD and NCTD operate it OSD-SAN. Essentially, it would be a Coaster train between SAN-OSD and a Metrolink train OSD-LAX/SNB, and may just take the place of an existing Metrolink and Coaster train on the schedule.
if they did do this, it would definitely take customers from Amtrak - being that it sounds like the fares would be essentially a Metrolink fare+a Coaster fare, we'd be looking at about half the cost of the Surfliner. If that were to happen, it may make sense for Amtrak to cut down on the number of Surfliners and specialize in express service and service to points north of LAX. Maybe even use the extra equipment to run a couple Surfliners per day all the way to SF/Oakland (a la Coast Daylight), though that would require the cooperation of UP...
I'd definitely appreciate this if they did it - would be nice to have a less expensive train option for getting to LA (and be able to use any Coaster station to get there). Would also like it if Amtrak had more express service and/or trips that went all the way to the Bay Area (though the latter may be wishful thinking).
I disagree with this assessment for several reasons.
First, the northbound weekday trains being considered are really early morning trains, which are before Amtrak even runs. They would cater more to commuters than to casual single-ride passengers. This would essentially be a new market, because right now commuters from south of Oceanside can't take the train to LA and get there before 9 am. So those would be passengers Amtrak isn't serving anyway.
Second, a ton of riders continue to choose Amtrak over Metrolink for lots of shorter-distance trips. Not everybody is going to San Diego. I don't have the numbers offhand, but the number of riders that only go to SNC or OSD is pretty big. Heck, some only ride to FUL. Amtrak fares are more expensive than Metrolink, so by that measure, Metrolink should have the market to themselves, but they don't.
Third, with rail-to-rail, a ton of passengers are already buying Metrolink tickets and then choosing to ride Amtrak, for the better equipment.
Fourth, Caltrans absolutely will not give up slots on the Surfliner corridor. They would not want to cut service. Nor would the LOSSAN JPA when they take over.
Fifth, the "express" service loses ridership and revenue vs. making the extra few stops. The couple of minutes saved doesn't attract enough extra riders to make up for the loss in business from the intermediate points. Amtrak and Caltrans both knew this, and past experience (San Diego Metroliners) showed it. Further, as long as you still have significant amounts of single track in San Diego County, plus a congested railroad north of the County Line, you're still going to have to sit and wait for meets. The only reason the current "express" operates is because some two-bit politician from Anaheim forced it on the corridor (and he's also the reason the train stops at Anaheim, which makes no sense, and not Fullerton, which would have made sense if the train made sense at all, which it doesn't). The express will go away soon enough, and that will (hopefully) be the end of that folly until a railroad is built that can actually accommodate express trains. For expresses to work, you need frequency so you're not sacrificing ridership at stops that are skipped. In order for that to work, you need capacity for expresses to pass locals. The SAN-LAX corridor has neither right now.