I'm not that sure it's a fake photo.
First, there appears to be a "bow wave", ripples at the front trailing off to the side. The train is probably moving but dead slow due to the possibility of hitting a section of washed out track, so there's only a minimal disturbance of the water.
Second, note that the engine's reflection is visible in the water. If someone just took a picture of a train and a picture of some water in 1927 and spliced the negatives together, I don't know how they'd get the reflection in there.
Given that we know the creek/river intersection was flooded in 1927, and that the railroad shown (SL-SF) did serve the area, it would seem more likely it's a real pic.
