What makes the crud and the dark smears when you wipe the rails by hand with a solvent of any kind is arcing. You can see it if you can completely darken your layout and run it. Nobody, but NOBODY...has rails that are perfectly level along the major axis, or even across from each other all the time. Variations in height with unsprung axles means momentary separations, often by short distances approaching a tenth of a mm. With the amperage and voltage available to the rails, those micro-separations cause arcing which is visible here and there....repeatedly. The arcing, fleeting though it is, is still plasma temperature for a fraction of a second, and that oxidizes pretty much anything you're likely to find on a layout, especially any dust, pollen, goo, lubes...anything left in even minute quantities on the rail surfaces or on your wheels' treads.