There are several desal plants around California and the southwest, but most are small (the newly built one near Carlsbad, and one proposed near Huntington Beach, being conspicuous exceptions). They tend to be suitable mainly for special situations where the alternatives are quite dire in cost and control, due to the high cost of building and running the desal plant the the difficulty of disposing of the resulting brine.

Santa Barbara built one many years ago, but as is common with such things it was finished just as the (previous) drought ended. Sensibly, they just mothballed it, but resurrecting it for the present drought seems to be costing nearly as much as the original (more than 10 years ago) construction. And it won't really supply the whole demand; it will just reduce dependence on imports to a sustainable (for now) level. Santa Cruz talked about one but the NIMBYs killed it; ditto for Monterey. Carlsbad (on the site of a former power plant) is just about done, but cost a fortune; the long-term contract for that water will probably bankrupt some ratepayers. I think there's a small one near Yuma to make Colorado River water usable in northern Mexico. Otherwise, they're hard to find.

Generally, to do desal, you need to be able to sell into an urban water system able to pay the high rates, have very cheap energy (e.g. Persian Gulf countries), or both. And it helps to have the ocean nearby for dilution and disposal of the brine (far offshore of course).

Again, though, railroad hauling to areas that might consider desal isn't really practical. Even a small desal plant operates continuously so the output is always available; rail is more like a big truck, suitable for a brief emergency use or supply for a small area, but not for large-scale long-term supply.