If you did want to reproduce that it would be relatively straightforward.
Airbrush the grime on and then "clean" the relevant bit of the loco with a minibrush, a cotton bud or paint brush moist (but not wet) with the appropriate thinners...mimicking the movements made by a cleaner - it could be a long handled brush done from sleeper level with some of the yellow panel also cleaned for better visibility as well as the headcode box?