Bruce,
I do not follow your 'theorizing'. Here is a link to NMRA RP-20.1:
http://www.nmra.org/standards/sandrp/rp-20_1.html
Based on your 50' freight car example(7" freight car); it should weigh 4.5 oz. A 80' passenger car( 11"-12" long) should weigh about 6.5 to 7.0 oz. I do not understand 'Since that weight is applied to trucks that are (proportionately) similar in length..' statement.
The problem with all cars weighing the same is that they are not 'in proportion' based on length. If you do not mix your passenger cars with your freight cars in a train, you can have a different 'standard' for your passenger cars. However, you may not pass 'club standards' if you belong to model railroad club. Clubs tend to make a big point about correct standards for things like wheel profile, couplers, & car weight.
Jim
Modeling BNSF and Milwaukee Road in SW Wisconsin
Theory is not better than practical. There are people who weight passenger cars heavier than theory, there are those who go lighter so as to put less strain on the locos. Either theory works. The more important thing is to have the weights uniform. I run mine lighter than NMRA RP with head end cars slightly heavier than cars at the end of the train. Since passenger caars run in more or less similar consists this works well. What this means is an 50' express reefer is heavier than an 80' coach at the rear of the train. What is more important than the weight is good trucks, wheels and couplers, no interference with the underbodies and uniform rolling qualities of each car to avoid stringlining
Looking through past threads, I haven't been able to determine if the same NMRA formula for weight, applies equally to passenger cars. Since they are generally longer, their weight would be substantially more than a freight car. Since that weight is applied to trucks that are (proportionately) similar in length, I would suspect the "generic" weight of a 50 foot freight car would be suitable for an 80 foot Harriman etc. How's my theorizing?
73
Bruce in the Peg