From a GM service manual:
Rail dust damage comes from the tiny iron particles produced from the friction between the train wheels and the track. It can also be deposited on vehicles if stored near any operation producing iron dust, i.e., steel ore yards. This dust can either lay on top of, or embed into the paint surface. It is usually diagnosed as bumps in the paint surface or rust colored spots in the paint.
Move the vehicle to a cool shaded area and ensure the vehicle surfaces are cool during the removal process. Thoroughly wash the repair area with Liquid Wash and Wax GM P/N 1052870 or the equivalent. Wipe the area dry. Clean the affected area with silicone, wax and grease remover. Perform the removal process according to the manufacturer's directions of the type of repair material used, Gel Type Oxalic Acid or Clay Type Non-Acid Based. If, upon inspection, some particles are still present, the process may be repeated. If the damage has been repaired, complete the repair to the entire panel. Polish the entire panel after the removal process. Refer to Clearcoat Repair Specifications - 3M Products or to Clearcoat Repair Specifications - Meguiar Products . If small pits remain in the clearcoat after all of the damage has been repaired, refer to Clearcoat Repair w/o Repainting .