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[quote user="Semper Vaporo"] Hate to drag this outta the past, but it is a start on a question that has been bothering me for some time.I have seen unit coal trains with consecutive cars that have reporting numbers that are all within counts of 200 or so... (I have seen 4 cars in a row with sequential numbers), yet the "LT WT" numbers ranged from 41300 to 41900 and the LD LMT ranged from 244200 to 244900 and I could not see any relationship between the two numbers on multiple cars... i.e.: a low LT WT number did not necessarily mean a high (or low) LD LMT number.I can see how each car can be weighed when it is empty and that value stenciled on the side of the car and I can understand how, knowing the type of truck under the car and what type and dimension of material it is consctructed from, one can calculate the capacity, but there appears (to this myopic untrained eye) to be large unaccountable variations in the numbers.Who is slipping in an extra 600 pounds of material on some cars (or leaving out 600 pounds of rivets someplace)?  And how can someone say, "this car can hold 244900 pounds of coal, but that one over there can only hold 700 pounds less"?Granted 600 or 700 pounds is only 1.5 percent of the LT WT, and even less of the LD LMT, but how can the LD LMT be determined with such accuracy, if they can't manufacture each car with the same accuracy for LT WT?[/quote]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.S. Hadid
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