There have been numerous proposals and modeling since at least 1980 on how to increase capacity on this corridor. I remember running into the late Byron Nordberg many times in Oceanside and several of his ideas and proposals are coming to fruition.

One thing to remember that that incremental improvements will usually only bring about proportional changes. In all likelihood most of the improvements are actually being used to add additional train capacity instead of provide options to keep trains moving when there are delays. Simply put, each train has a window to operate in with preplanned meets. If the train is delayed by mechanical issues, station delays or BNSF/UP congestion - they miss their window and the dominoes start to fall as the dispatcher tries to figure out how to run a train late. During the rush, a train that is late outside its operating window is actually like trying to run an extra train where there is only limited open capacity (open sidings or multiple main track for meets).

I can see them designing additional track and signals for 110 mph, but until there is enough of it to operate across all of Camp Pendleton at 110 instead of 90 there is no advantage. I don't think any of the current Amtrak/Metrolink/Coaster engines are geared for over 90 at the moment.

The politics of the whole situation have and will prevent timely improvements. For instance, when the new Solana Beach station was built, the dirt from the grade separation grading should have been use to extend the grade for a second track across the adjacent slough. Instead it was hauled to the beach to replenish sand, and then the residence complained it was brown instead of white sand. Many of the adjacent residents object to any additional trains.

Eventually most of the track between San Diego and Fullerton will be two main tracks, but it will continue to be a slow and painful process. Once most of the easy sections get a second track, the rest will become very expensive due to crossing sloughs, bluffs, limited right of way to adjacent homes, bridges and Miramar hill. I'm not sure if there is an easy answer to Miramar and no doubt the tunnel option will again be considered. Urban encroachment is quickly limiting other options involving heavy grading or realignment.