Actually, I think this is one of the best NMRA standards out there, and is equally applicable to all flavors of "O" trains, both scale and traditional. The reason it works, regardless is, it is based on car length.
In the 3 rail world, we may not need to be quite as precise. Our oversized flanges provide us a little leeway, but it is still a good idea to add weight to any car that deviates grossly from the standard. Flat cars are a prime offender.
Just for fun, I grabbed one each of the Lionel 86' box cars and 89' auto racks off the layout and threw them on the scale. Both weighed in at 25.3 oz. Doing the math according to the standard, the box car should weigh 26.5 oz and the auto rack 27.25 oz. I don't plan to add weight to either, that's close enough. Beside, adding the extra weight will probably not gain any advantage, and adding enough over the length of a train will eventually force you to knock a car off the end because the engine can't pull it.
BTW RRMAN, measurements should be end to end on the car body, not knuckle to knuckle.
