The B&O T-3 Mountains were rebuilt from older power, Mikes I think. To do this, the RR had to lengthen the boilers, which gave the T-3's a somewhat lean look between the stack and the sand dome (or is it a sand box?). I remember an old Trains article where Dave Morgan advised that not much else from the original engines was reused........
Much older, but the NYC rebuilt some 2-6-2's into Pacifics. The 2-6-2's were very fast but somewhat unstable (if you believe the various narratives) and this was attributed to the two wheel lead truck. The resultant Pacifics were not deemed successful.......
The MoPac rebuilt some older engines, 2-10-2's I think, into high speed Mountains. These were very successful.
