The law required that scales used in commerce be checked on a regular basis. Usually as far as the railroad was concerned most shipments went by the weight on the shipping bill but bulk commodities had to be weighed. Most every yard of any size had a scale track where cars were weighed as they passed over in a string. There were a lot of scales to calibrate and many railroads had these test cars.
I remember when I was trucking that I stopped at a dealer in “used” cardboard. Having just loaded a box car full of corrugated one of the employees was soaking the interior of the load with a water hose completely saturating it so water ran out in a minor deluge.
I asked the guy why he was doing that and he told me that the cardboard was being sent to a paper mill that had bought it and the car would be weighed by the railroad and the customer billed for product by the weight. Enough moisture would remain by the time the car was weighed to add a considerable bit of weight to the load thus increasing profit.