To sneak into the factory undetected in American Fathers 2 for Eagle Flies in Red Dead Redemption 2 walk up to the cart as it is waiting for the train and jump on the back of it.
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Ride the wagon until it stops inside the oil factory compound. Get out of the wagon and sneak across to the back of the factory and go inside.
Follow the guard walking to the side entrance and keep going to the stairs at the end of the factory floor.
Go up and stick to the left side of the upper level and go around to the back door to see the factory manager.