Those gray UP units are radio-control cars, made out of retired B23-7s (ex-MoPac, I think). They have no prime mover themselves, but m.u. with other locomotives that don't have RCL equipment so that the whole lashup can be run from beltpacks. They're very common on UP locals in southern California. A good place to check them out is in Anaheim, where they are easily viewed from the street (Broadway?) when parked.