Tonight I primed and painted the loco frames and in addition cropped the motor shaft so the 1424 will sit comfortably inside the Smokebox.
Having performed these tasks, I placed the body on the chassis to check the fit which appears to be bob-on.
Some hint of the weight and mass of these beefy Locos is transmitting itself into this model; I think this is evident from the sit.
The model weighs a (metaphorical) ton and between that and the motor/gear combo I suspect it will pull anything I care to fling at it. Testing day at Wharfeside may be entertaining!
Max Stafford's Kennel 
Maybe an optical illusion, but is there a mis-match between the wheels and the splashers?
Having checked the model after you said that, it seems about right. I think perhaps it wasn’t sat quite right on the chassis in that photo; an effect exacerbated by the camera lens.