I noticed that building, presume it was a pump house for pumping water out of the quarries. I read somewhere a comment about steam pipes around the quarry area, perhaps compressed air though for tools?
The Engine House has gone from the 1899 map. My images are Geotagged, I will check the ones in that area later.
You will notice that the tracks do not go to the Engine Shed on that 1873 map but do on the 1899 map.
The maps are better quality on the NLS website.
Martin Briscoe
Fort William