I discovered the cab number stickers on the sides of the headlight of a Rial King engine had fallen-off because the adhesive failed. Luckily I found the stickers on the tracks, and I reinstalled them using a very thin film of 2-part epoxy adhesive.
I advise you to check the headlight number boards on your Rail King steam engines. Not all Rail King engines have the cab number stickers. Look for evidence of glue residue, and check the catalog photos of your engines to determine if it had stickers.