The links below should give you everything you want to know about rail dust and paint contamination along with the "Proper" way to correct the problem without degrading the paint finish with methods such as oxalic acid washes, buffing, wet sanding and yes clay.
http://www.fk1usa.com/decontamination.htm
http://www.fk1usa.com/Decontaminatio...20Brochure.pdf