The US Gypsum 3 foot operation in Plaster City is alive and well. I live in Phoenix and usually travel to San Diego 3-4 times a month. Once and awhile I get tired of taking my shoes off and getting out my labtop at the airport screening area, so I drive. You can see the US Gypsum plant from I-10 west of El Centro. I usually make a detour off of I-10, loop through Plaster City and get back on I-10. It is usually hit or miss, I am sure they have a pretty set schedule, but I have seen a few trains, it is not Cumbres, but it is pretty interesting.
I have snapped a few pictures, from the road, I have never been hassled, but I keep off the plant property. You can watch the unloading process from the road, and view the engines in their servicing area from the road. The engines have to cross the hiway to get to the train and unloading area. I have got a couple shots of the engines crossing the hiway. If you are in the area, it is worth the time to try and see it. Plaster City is not a town, it is nothing but the plant, offices and other operations bi-sected by the road.