Adding soot effects to track?

[RobertInOntario]

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.

The pic that I posted is from a modern-day steam heritage line in Somerset, England. The line is mainly travelled by steam locos although they do have a few diesels running occasionally. In addition to that pic, I've been looking at a few photos in my British 1950s/'60s steam era books (steam ran until the late-60s in Britain) and these pics show a darkened area on the tracks, especially in the "4-foot". So would that have been caused by oil & grease coming off the locos and freight car wheels?

The darkening effect in the pic that I posted is almost extreme -- it's less noticeable in my books (mentioned above) but still there.

Rob