Usually, at the end of the line, the cars were sent off to the coach yard where they were cleaned inside and out. About the only area on a passenger car that would get dirty is [maybe] the center of the undercarriage, but anything on the sides and ends would definitely get washed down. At LAUPT (LA Union Station) I'd routinely see strings of Superliners going through the wash rack in the coach yard (ex ATSF) just southeast of the terminal near the I-10 Freeway.
One thing you would get is oxidation of the paint or stainless steel. Oxidation of the paint can be simulated with a light spraying of thinned light "dust" paint. Oxidation of stainless steel is tricky as you don't get a lot of rust except over a long period. By then, it might get hit with aluminum silver paint.
