I found another photo of the lashup that is shown in the first photo. It's not easy to see, but this time it's an FM-F3B-BL2 combination. The caption on the photo is:
"Forest Hill interlocking; Bill Raia photo"
My guess is that it was taken about the same general time the first photo was taken. Based on the scene, it looks like all three locomotives were operational.
George L.