Hi SD40-2,
Yes i model American here U.P of course
BNSF norfolk and southern deisel with PRR passenger locos an intermodal layout in the planning but N gauge due to space restrictions, mainly Kato and life like locos in DCC cleaning the wheels could be the solution also the track especially if it happens in certain areas you notice.
The best way to clean your wheels it to get a piece of kitchen tissue and use some Isopropyl alcohol, put a small amount of alcohol on the tissue (only enough to wet it) and lay across the track get the loco on top of the tissue the wheels lying as usual on the track and gently apply power hold the loco letting the wheels spin on the tissue , when you see black marks form turn the tissue to a clean part repeat running the loco on it and repeat until the tissue is clean finishing with a clean dry piece of tissue, if you have a piece of track seperate to the layout powered then it keeps any dirt away from your other tracks of course, do not put a load on the loco though by holding it too long while running.
Hope this helps ..............