This is not a criticism but a suggestion. When painting your diesel models, I have found they are much more realistic looking if you darken all grills, exhaust fans, engine exhausts, and air vents. Also the walkways should also be darkened; I just found out about that one myself while studing my photos of the Firsco GP40-2's. I have had good luck using grimy black. Watch out for the E and F units, they are dark behind the Farr Grills. Look at any prototype photos and you will see what I am talking about; this is not waethering but detailing.
That's a great tip and an easy way to increase realism. If you use a wash of diluted black and then quickly wipe the surface with a paper towel the raised areas remain light which adds to the 3D effect. Diesels are more challenging to weather and detail than steamers in my opinion. Merry Christmas.
