I have several more photos of that factory, for what it’s worth.
You said the maps don’t show any railroad tracks in the area. But isn’t this a railroad bridge?
Here it is entering the factory:
A footpath goes under that bridge and about three others:
When you come out, you’re here:
Here’s a closeup of one part:
and of another:
This next photo shows the river you saw on the map. The factory is just to the right. Note the weird blockhouse-like thing on the far side.
Here’s a closeup of the “blockhouse:”
Is any of that useful? — Flyss