Wish everyone could run their trains in the environment David has. Then you can keep your layout, track and wheels clean. On the gunk issue, the first question you have to ask yourself is what is it? I know that answer on my layouts. Everything in the air that mixes with oil from lubricating the locomotives as well as oil from fingers, never mind spillage from friends eating and drinking around the layout. You have gunk. Again as those in the cleaning profession call it, Environmental Build Up. I would agree! I've seen plastic wheels wear a groove in them. A caboose on one of the San Diego Narrow qauge model railways showed grooves worn into its wheels. They were using a sharpheaded homemade rail and is most likely the culprit causing the wear. No idea if they were Delrin by MTL. I doubt it. I don't they they made any back when.