Here's some second-hand information to fill in the gap while we wait for someone with more solid info than I have to jump in.

1) Santa Maria Valley - from what I have been told, they moved out of the engine house in Santa Maria and or now in an inaccessible siding outside of town. The daily train to Guadalupe is fairly easy to shoot however as it crosses a couple roads and can be seen at the UP Yard in Guadalupe.

2) Southern California RR - I believe that this is the switching RR at Plaster City, yes. My one time there, I didn't see them, so I can't help. If shooting the Plaster City narrow guage mine train while there, I would recommend introducing yourself to the front office so that they know why you are there.

3) Carrizo Gorge Railway - Been lots of reports that this operation has shut down between Campo and Plaster City, but I am honestly just typing stuff that I read on the internet... I have no personal knowledge. I think the Mexico stuff is still running, but don't know for sure. I do believe that the cool engines are still sitting in Jacumba (at least the Alco switcher is still there, not 100% certain on the F units).