I'd make an educated guess and say it's down for wheel work. I've seen Cascades power come down this-a-way a few times, as well as having seen Coaster F40PHs running through Orange County for a wheel-truing as well.

Also, we had some...issues with our regular NPCU, #90230. Back in September it was bringing up the rear of AMTK 572 south when it developed 4-inch flat spots and brought the train--and a small excursion with the Silver Splendor, coupled right up against the power--to a grinding halt near Capistrano Beach. Lemme tell you, even when they grounded down those flats, 90230 still was clattering along when it blasted through Irvine later on that evening. #90208's been pinch-hitting for it these past few weeks, but it looks like it's bearing the marks of sustained service and deferred maintenance.

I wouldn't be suprised to see 90340 working the Surf Line...