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The testing site is one of three sites where Hornwright Industrial Mining Company tested their mining equipment.
Hornwright testing site #04 is a small building surrounded by abandoned train cars which is located up the hill from Brim Quarry. Inside the building are two rooms, one of which is a makeshift workshop containing broken robots. There is very little of note at this location other than assorted junk items and a very empty refrigerator.
Hornwright testing site #04 appears only in Fallout 76.