Ford actually has a recommended procedure for rail dust removal and it has been around about 14 years that I know of for sure, and maybe longer. It involve an acid wash, then regular wash a polish. It doesn't involve clay bar, because they contractor they hired did not feel that is was effective.
I've done an acid wash once before on a vehicle with etched water spots. Definitely not something the average joe can do and you really need two people to do it right. Letting it sit for too long can really screw up your paint.