Does anyone know if earlier 4-8-4 Northern shells (I have one of the UP "grey" shells as well as one of the newer ATSF shells) will fit the 4-8-4 Norfolk & Western Class J ("DCC-ready" version) frame?
Neither of the locomotive mechanisms for the UP or ATSF shells runs very well :( and my understanding from various sources is that the "DCC-ready" Class J's are Spectrums and run very well.
Failing this, would the Con-Cor GS-4 4-8-4 mechanism accept one of the shells mentioned above?
Note that I would prefer to stay with the Bachmann mechanism.
I don't know which versions of the Bachmann 4-8-4 Northern you've tried (there have been at least six), but the most recent version has a split-frame design and runs quite well.
(http://www.spookshow.net/loco/files/bach484mech6.JPG)
-Mark