Hi tubab
I cant see where you can make a comparison between the D51 and C57 trains that are currently available as each was designed to do totally different jobs on the Japanese railway system. The D51 was the freight and heavy goods carrier and the C57 was for general purpose and passenger use. So both were designed to give the maximum in there given roles.
As with all heavy engineering company's around the world standardization of parts is a very common practice and was even done here in the UK on our loco's. So to see comparable parts on both engines is a normal practice but it still will not make them the same.
For instance the water feed tank is on the top of the D51 smokebox whilst the C57 has the same unit on the front deck both units are the same but just in different positions and a classic case of standardization in Engineering.
As for your statement in a previous post that parts can be used for either engine build if you look closely at the water feed tanks there mountings are different as in the D51 having a curvature to sit neatly on the smokebox in front of the smoke stack the water feed for the C57 has not been given this feature because it has no need for it.
Therefore although the parts are of the same unit these parts are not the same and in no way could be used on another kit apart from the one they were designed to be used on.
Looking at either build to help in the build of the other has its uses as a comparison of some parts only as although the parts may look the same most will have a different mounting and attachment point so I would be very careful of using any other build than the one that is for that build
Regards
Andy
Current builds:-C57,Zero, Lamborghini Countach, Caldercraft HMS Agamemnon,Robi,R2-D2, MFH Cobra .