I'm going to 'weigh in' on an -entirely- different scale ...
Modeling "scale weight" serves no purpose. It sounds good on
the surface but you have to ask your self the question "why"?
I suggest you weight your freight cars for excellent operation
and forget -entirely- about whther or not the weight is 'scale'.
That said the NMRA is a good starting point - because it
addresses the 'weight per unit of length' of the train and
improves the operations. For my own trains (HO) I usually run
them at something around "NMRA plus one ounce per car" because
I have found that the extra weight translates into improved
operation without greatly affecting the number of cars you
can pull up a typical model RR grade.
And let's not forget that the relative locomotive weight on
drivers, materials used in constructing the cars, whether or
not the cars have truly functional springs in them, whether or
not you have draft gear that cushions the slack action, etc.,
etc., etc. ... all directly affect what you should/shouldn't
use as a standard for your cars in order to achieve better
- YMMV ... Jim in San Jose