Adding soot effects to track?

[Sumpter250]

Steam era track would have had little or no "oil" down the middle of the track. There might have been some ash, from the ashpans, and any "leakage from freight cars" ( grain, cement, coal, etc. ).
There would be one other "weathering effect", and that would be on the up-grade tracks, there would be a buildup of sand, that would be heaviest on either side of each rail, and between the rails. This becomes particularly obvious with two track mains, where one side is consistently the up-grade, and the other, the down-grade track.