As an example of prototypical weathering...from TO website
The former Codelco (the Chilean national copper mining company) Potrerillos line, now operated by Ferronor, sports some of the dirtiest units I've ever seen (the UP flag units are spotless in comparison). Here are a couple of photos taken in March 2007, along with one of the units in it's light green livery in 1992 before dirt set in.
The line in about 55 miles long with a ruling grade of 3.5% from the valley floor at Llanta up to the mine. Near the top is a series of tunnels, which keeps the units covered in exhaust soot....there is just no point in trying to keep the units clean.
UNweathered version....
WEATHERED version....
Walter M. Matuch