Cut 'n paste from another forum/site:
The maximum gross weight is the number that matters. It's an admittedly arbitrary number within a certain range (there's no practical difference between 286,100 lbs and 286,000 lbs.) but it is the number that is established by agreement among the railroads. The line has to be drawn somewhere and that's where it's drawn. The car is placed on a scale, and whatever it weighs is subtracted from the desired maximum gross weight, and that difference becomes the capacity.
Steel castings and forgings (truck sideframes, wheels, etc.) have small variations in wall thickness, etc. Steel shapes and sheet have dimensional tolerance range according to their specification. If you want to buy 0.5000 sheet steel and not 0.5010 sheet steel or even 0.5001 sheet steel you can do that, but you will pay a tremendous amount of money for it.
Consult your ANSI standards for sheet steel and shapes, and AAR standards for steel castings and forgings. They will describe to you the tolerance range that is the standard for the steel manufacturing industry and the railroad industry, respectively.
The variations tend to mostly cancel each other out within a narrow range.
If you want precision you will pay for it. No one needs precision in a freight car body. For each order the manufacturer provides a guarantee that the cars will fall into an agreed-upon weight range, a range which is derived by the manufacturere from experience. If you wanted every car in an order of 1000 cars to weigh within 100 pounds of each other you would pay a huge premium to the manufacturer. Will you realize that premium in car efficiency or rate reductions? Absolutely not.
Manufacturing has tolerances established for everything. The tolerances are only as tight as they need to be for the function of the product, within economic reason. At a certain point the cost of manufacturing to tighter tolerance exceeds the value realized from the tighter tolerance, and that's where you establish your tolerance range limits.