This is chapman's mill, lots of great things to see inside this building! there is also 3 other buildings nearby. You access this by driving down road next to rail (big fence by parking lot), and you can't explore this from within the park trail.
The mill has been reinforced very nicely with beams and bolts in the right places... it is totally safe to go inside and check it out.
The mill has limited hours of operation, as they close the gate here, but you can still hike inside the conservatory and see the rest of the building!