Whatever..., it's still caused by micro-arcing.
Actually, there is something that can be done about it, other than going to onboard batteries.
To minimize, you want to have as many wheels and sliders in contact with the track as is possible so that each contact point realizes minimal current.
Also, you want to keep the wheels and track as clean and smooth as is possible, for the same reason.
If you are running plated wheels, eventually you will not be able to easily clean them as the plating flakes. When they get to that point, I turn the engine over, power it up, and use an X-acto blade across the treads like a "lathe" to remove the flaking plating and the black residue. This works a lot better and faster than alcohol on a cotton swab. Afterwards, I use the alcohol/swab.