Really test whatever you are going to try. Some paint can be irreversibly damaged by any water... Some colored older litho will wash off with water alone. TEST, TEST, TEST. I prefer white Ivory or Dial liquid soap in minute quantities for damp cloth cleaning. Watch those high spots like rivet heads, panel offsets, and roof catwalks. The edges have the thinnest application of litho or paint.
Some mildews will come off with a cloth (or toothbrush) application of synthetic oil (paint and plastic tested of course) and a thorough cleaning after.
People have used protectants for years and many have had good results. A good cleaning, perhaps polishing with a high quality automotive low-cut compound is all that the paint really needs. Do not use Armorall. On your car... or your trains. It will trap dirt. Same with WD40.
If you feel that it HAS to be protected, use pure carnauba wax. I have not tried any of the new silicone materials as yet.
This is all just my opinion.
