weathered admx and uelx covered hoppers

[h44]

a lot of the yellow adm or uelx center flow covered hoppers seem to have a heavy coat of blackish brown streaks down the car sides what cargo creates this weathering?
ty
mark

[cv_acr]

Just general road grime, dirt, rust and soot washed down the sides by the rain.

http://canadianfreightcargallery.ca/...x100159&o=admx

ADM handles a lot of grains.