I still don't think there is true scale in standard gauge. As proof, look at the difference between 500 series freight cars, 200 series freight cars and American Flyer cars. They are all significantly different sizes. If you were trying to understand scale, I think it would depend on a specific model and manufacturer. Even then, scale would be very subjective. I would treat building a layout with standard gauge more like creating artwork. Use your imagination and other artistic tools, such as selective compression and forced perspective. I think this is part of the fun and what I really enjoy about tinplate. I even mix O gauge and standard gauge, but I usually run them on different levels of a layout or space them in a scene to create an appearance of distance. There are some very interesting layouts that have been built by mixing scales.
Alternatively, you could ask the question of which manufacturer/model is most prototypically to scale based on the track gauge. I'm not sure I have the answer, but I would say it is not Lionel or McCoy. Maybe it is later years Ives, but I don't have any proof.
George
