I believe that the clear coating is simply polyurethane, such as oil based Minwax. The rods and bright work on the South Park locomotive received the same treatment. Polished brass held up quite well as did the steel.
Polish the surface, clean well with a good solvent cleaner that evaporates without any residue and give it a coat of the Polyurethane. Throw away foam brushes work well and leave no brush marks.
I am sure that someone from the museum will correct me if I am wrong.
Rex Beistle
NMRA RMR
Estes Valley Division