I was having trouble with my locos getting around the layout smoothly. They kept getting stuck. So, did some research and bought a Brightboy. Scrubbed my rails shinny then ran the trains. Better, but still acting up. I thought it was my electrical, I thought is was my locos - if I had any hair I would have pulled it out in frustration. Then I happened to run my fingers across the top of the rails and found there is some sort of black, possibly oily, substance on the rails. Some alcohol (for me and the rails) and a rag, and cleaned all of the black stuff off. Now, trains run like they should.
The question? What is the black stuff I cleaned off my rails?
It only takes a tiny amount, plus the wheels of a few passing trains, to foul a lot of rail.
Yep, my rails ARE new and your description makes sense. It also occurs to me that part of my layout sits directly beneath a heater vent. So, when then the wife cooks or uses hairspray, there's a chance a thin coat of black stuff will continue to build up.
Guess i better keep plenty of alcohol and rags around...