Adding soot effects to track?

[RobertInOntario]

I've recently been adding some soot effects to my two small layouts by brushing on some black/grey powder (kind of like a weathering powder) that I bought at an LHS.

The effect is quite good. I've been looking at some pics of old steam & diesel railway lines trying to find out the areas that would have been the darkest. I think the main line would have been the sootiest where the locos would stop frequently and that some of the areas further down on the sidings would not be as dark. Does this sound right & can anyone comment further?

Thanks,
Rob