I asked this question on the Norfolk and Western RR list, at Yahoo.com, and have only recieved one reply so far. If I hear anything different, I will let you know, but this sounds about right. Norfolk and Western began using the large NW in 1971. (They call it the F6 paint scheme). It would have taken several years for the railroad to repaint their locos, so you might easily have seen an old paint job in 1978 or later. The person who answered could not tell me when the SD35s were repainted, exactly. It could have been any time after 1971. The railroad would not have stopped operations to bring all the locos into the shops to repaint them...it would have been done as the locos came into the shops for scheduled, heavy repairs. If you are modelling any time in the 1970s, go ahead and mix the paint schemes.