You find a static drawbar-pull number (X pounds @ Y volts) in many HO engine reviews but I can't recall seeing a published dynamic tractive-effort characterization (drawbar-pull x speed) for any engine in any scale.
Every so often (usually in HO) an operating dynamometer car like the MOW prototypes comes up. To date every one I've seen has been crudely implemented using the spring method (essentially a fish-scale) that Dave describes. And I've never seen a dynamometer car that indicates not just drawbar-pull but actual power (scaled Horsepower) which accounts for speed and I think is what many people associate with load capability.
As I see it, there are too many variables to come up with a user-friendly number. Your assessment of differences in car types is a good one. Then, for example, smoke unit power goes through those same rectifiers and I can't imagine a manufacturer publishing that you lose 5 cars (or whatever) of tractive effort if you turn smoke on! And practically speaking, I can only imagine the can-of-worms it opens up with MU configurations. If a manufacturer stated a particular engine pulls X cars, does that mean 2 engines can pull 2X cars?
