I used Phoenix San Diego as an example because for some reason, it appears to be a busier point to point air corridor than Phoenix to LA, and because of my supposition that right of way acquisition would be cheaper.
Does Phoenix's airport act as a hub for flights involving San Diego to various other places to which LAX has direct flights that wouldn't require going through some other hub, perhaps?
You're probably right--PHX is probably a "gateway" to the rest of the country from SAN.
PHX is a hub for US Airways and a focus city for Southwest (who doesn't really have any "hubs," per se).
WN has a sizeable operation at LAX, so there are more nonstop flights from LAX to other cities than from SAN, where many flights probably route through PHX.
LAX is not a hub for US, but again, it being a larger operation, there are probably more nonstop flights from LAX to other cities or to US's other hubs (LAS, CLT, PHX, BOS, etc.) that don't route through PHX.
Would be interesting to find the numbers broken down by airline. That could help the analysis.
Note that if a large number of people transiting between PHX and SAN are connecting from other airlines, those passengers will be much more reluctant to switch that final journey onto a train, since much of the convenience of the train (lack of security, city center station, etc.) will be lost, as they'll already be at the airport and behind security.
However, this could potentially be overcome if the train were fast enough, had an
in-terminal station at PHX airport, and if WN and US decided that they could redeploy their assets elsewhere more profitably (as has happened with the longhaul carriers in Europe) and cut back air service on the PHX-SAN city pair (thereby reducing convenience and raising fares, driving people to the train), perhaps even booking the train as a codeshare.
It
can be done--Lufthansa sells tickets on DB between Frankfurt and Stuttgart, even though it also serves Stuttgart by air. I did STR-FRA by ICE train on a LH codeshare in 2001 (by choice, of course!), and the transfer from train to terminal at FRA was absolutely seamless--just up an escalator and left, about the same distance as walking to the AirTrain station at EWR. (I think we may have even pre-checked our bags in at the Stuttgart train station.)
That seamless, and convenience-oriented Americans just
might do it. Any harder (take an AirTrain, shuttle bus, or whatever), and people will just fly.