About two months ago, I put my first and only N scale train on my lay-out, and there have been no problems. Which is odd, because I put it in a hard-to-clean area. Today I took it down and scrubbed the tracks with rubbing alcohol.
To my surprise, the tracks were almost clean, and the wheels on the loco were clean. My HO tracks always show grease and dirt, as do my HO loco wheels when I scrub them.
Any scientific explanations for this phenomenon?