Weathering black engines requires a slightly different approach than those of color. Black washes and dark colors don't really achieve much. A much better approach is to use lighter colors to represent dust, dirt and fading. For reference, here is a great picture of a weathered black unit by noted modeler Jim Six ....
Mark.
¡ uʍop ǝpısdn sı ǝɹnʇɐuƃıs ʎɯ 'dlǝɥ
Any tips on eathering a black GP40-2 style loco?