My rails are metal, aluminum I believe. I clean them with rubbing alcohol on a scrap of batting and that usually picks up some black residue. Then I crumple up wax paper and rub it back and forth on them. I also clean the wheels of my carriage with rubbing alcohol on a Q-Tip and clean the rubber on my encoder wheels the same way. My carriage wheels are some sort of plastic, but I am only waxing the rails which are metal. I don't know if waxing plastic parts would make a difference.
This may not work for every rack. I do know that Jamie Wallen of Quilters Apothecary currently quilts on an Innova which is probably why he recommends waxing the rails. I also quilt on an Innova. I can't see buying a special wax for that when the wax paper works just fine.