I posted a little while ago about rear bogies, which has led me to look a bit more closely at the relationship between “scale” and “semi-scale” Hudsons, as far as I am able.
The first conclusion is that the “selective compression” in length seems to be concentrated around the firebox, behind the rear driving axle (this becomes obvious, from the drawing someone was recently good enough to post)
The rear (trailing) bogies in Lionel “small” Hudsons, seemed to be an important part of the different appearance. Looking at my recently acquired MOC era “small Hudson” and comparing it to the “train set” Polar Express Berkshire, it is apparent that the rear bogie is quite different. The Berkshire has a different bogie, with longer wheelbase, higher sideframes and different wheel diameters (13mm and 17mm) compared to the twin 14.5mm wheelsets on the Lionel loco.
The K Line Hudson also has this feature. I don’t have a good photo of it, but it can be seen here;
So, one key feature is the rear bogie, which needs to match the line of the firebox skirt for best effect. The K Line also has undersized pilot wheels, but nearer “scale size” than the Lionel loco.
This is interesting, because I saw this older Williams loco, described as a “Scale Hudson” on eBay;
This clearly has the extra length in the firebox, but the trailing bogie (despite having different wheel diameters) seems quite wrong in overall effect. THIS later one, also currently on eBay, is clearly “semi-Scale, short firebox” configuration - but the line between the bogie and firebox seems much better executed
So the conclusion seems to be, and I apologise to those who knew it all along, that the rear bogie is very important in the overall balance of the model; and a well-executed rear bogie goes a long way towards disguising the “missing” length in the firebox.