Adding soot effects to track?

[Russ Bellinis]

I'm wondering if the use of the line determined the weathering on the track more than whether it was steam engines or diesel. If one line was dedicated to passenger train operation, but the other saw a lot of coal hopper transported, much of the dark weathering might be due to coal dust falling on the tracks even out of the bottom dumps in the hopper to give a general black, sooty look to the entire track. A passenger train would not have anything except the locomotive to drop smoke and soot.