An easy way to enhance the look of any diesel locomotive is to blacken the grills and vents to provide more depth.  You can use a black wash to bring out the grill or vent details.  Weathering powders are extremely effective bringing out locomotive vent and grill details.

Hopper Weathering Clinic III
By Bill Mock
January 2010

OK, I gave the inside of these hoppers a wash of Engine Black below the coal line. They [ ... ]

Is so awesome!! Not exactly prototypical, but Milwaukee Road none the less! So glad someone made this!!

 This is Micro trains Line SD40-2 Santa Fe model which I customized with working ditch lights.