There is black residue on the top of the monorail.
Black flecks on your rail are normal. They are a combination of dirt, perspiration, molydisulfide (a lubricant from the plastic rollers) and a very small amount of the nylon plastic that the rollers are made of. These four components are subjected to the mechanical action of the rollers and can dry and harden on the rail. To remove the black residue, clean the monorail. Wipe the monorail with a cloth or non-abrasive scouring pad after use. You can use soap and water or any glass cleaner. Do not use mineral acids, bleach or coarse abrasives.