Having never had the confidence before to look at and hopefully cure a poor running locomotive I have now cleaned the wheels on two previously poor runners which to my eyes had clean wheels to start with before going any further and seeking advice. The results have been quite remarkable. The point of a post on a subject that has been done to death is just that I now believe " clean" means far more than I previously thought necessary , especially as I run dcc. With regard to track cleanliness the same now applies but here I still have a problem. Why does a track that was " clean" two days ago and has had no running on it now have a black residue which shows up on a white cloth? Does nickle silver track degrade constantly or is there some other cause?