First - Climate change, real or fiction. In my opinion real, but not the same reality as the environmental experts. It is a proven fact that through history our earth has gone through many cooling and heating cycles, with attendant sea level changes. As a kid growing up in that region, I never heard of El Nino or La Nina yet we still had wet and dry years. I will say that recent history seems to be far drier, but even California Southern learned the hard way how a arroyo worked.

As far as the Del Mar bluffs, you can't stop erosion. I can remember as a kid the San Luis Rey valley flooding clear across - the orange groves and tomato fields in the river bottom just got a fresh load of silt deposited that was disced under as soon as it dried up. Now - all the groves and fields are gone replaced by housing tracts who will no doubt be surprised when it inevitably floods in the future. Just like those that build (whether tracks or houses) on the beach or bluff, erosion happens - it's a simple fact of life.

Yes - bypassing the bluffs will be extremely expensive, but much of that expense is due to NIMBY interference, political pandering, environmental concerns due to the sloughs/estuaries and outrageous real estate values for any additional land to accomplish this. The ground along there is a brown sand that is basically an incomplete sandstone. Assuming seismic issues can be resolved, this is likely an excellent use of a TBM (tunnel boring machine) since they have been used to even drill the subway by the La Brea tar pits. Unfortunately most projects of this size have too many "additions" such as public art, sound walls, other remediation, etc. that have nothing to do with the basic project. In addition, it is well known as on projects this size the contractors typically bid at break even (or below) and plan on making their money on the change orders (many to cover their or the project's incompetence).

There is another benefit being overlooked. Properly presented, those with the million dollar beachfront hovels will gladly support getting rid of that nasty awful train that interrupts their beachfront view and beach access - as long as it doesn't cost them anything. Won't they be surprised when no one is left to pony up the bucks to deal with the bluff erosion once the tracks are gone! Years ago a lady was walking her dog along the tracks on the bluff. Fifi dog decided to stand its ground in the middle of the tracks and bark at the oncoming train. Lady decided to risk her life to save her stupid dog with the predictable result. How many trespassers have been killed in this section?

Sooner or later, it will be necessary to install a second main track all the way from the BNSF to San Diego Union Station - they've got a good start on it. This is one of the areas that a second track is not really practical due to the geography, so while expensive, the second bore or alternate bypass route is needed for the second track. The time saving is somewhat inconsequential compared to the other potential benefits.

Wasn't one of the options considered at one time being a cut and cover tunnel adjacent/under I-5? If you overpower an electric lightweight train it might work, but I don't think the grades would allow freight.

California has missed far too many opportunities. I remember when US395 was just a two lane to San Diego. When they replaced it with I-15, there was an allowance for bringing the San Diego Trolley up the median to Escondido. But "smarter" people prevailed, and instead a directional flow carpool lane was installed. That became overwhelmed, so they rebuilt everything again to a make it a four lane (2 each direction) separated carpool lane. And each change seemed to involve adding more regular travel lanes so what was 2 lanes, became 2+2, to 3+2+3 to 4+2+2+4. For what they have paid in freeway expansions, they could have installed one hell of a light rail system.