I will check the wheels now as I did happen to notice that a trailing coil car has dirty wheels. I had to take it off the rails as the KD knuckle spring had fallen out. I have been running for a few years without any track maintenance issues. I even tried to clean the rails before and got nothing.
Now I'm starting to see dirty track in some areas. I hate to say it that I'm regretting the ballast! Hopefully it won't be a big reoccurring problem?