Adding soot effects to track?

[RobertInOntario]

It's not really soot but oil & grease falling off the rolling stock especially locos. It would tend to be down the middle between the rails. It would be very concentrated where ever locos would be stopped like around engine facilities.

Yes, good point. I think it's oil, grease, diesel effect.

Here is a pic (taken from an actual steam railway in England that I visited in 2009 -- it's awesome!) that I've been using for reference. I think this is a great shot and I well remember this section of that railway.

Cheers,
Rob