I've some AF Wide Gauge brass plates that need a good cleaning and polishing. Any ideas on the best way to approach this without destroying the black?

I found this stuff while doing a search for safe brass cleaners for antiques. Although I've never used it, it says it is non-abrasive and gentle.

The ones I have cleaned have been off the engine/car because of a complete re-paint in progress.

I take a piece of heavy card stock, as found on the back of a note or legal pad, put some Brasso on it, and place it on the bench. The plate is then rubbed on the card stock as necessary to polish the brass. Due to the card stock's stiffness, the Brasso is not rubbed against the black, it being recessed.

After cleaning with a soft cloth, I usually do not overspray. When I do, I use a clear acrylic lacquer.

I believe hand-held card stock with minimal Brasso applied could be used with the plate on the engine/car if one were to mask the adjacent painted surface. A mildly stiff brush would probably be necessary to remove dried polish from the edges of the plates.

I can see the cardstock idea working with Lionel plates but the AF plates are reversed. That is, the black is on the surface and the exposed brass is recessed (unless my eyes are completely failing me...).

I've just checked several O and Wide gauge steam and electric engines, several freight cars, even an Ives/AF transition car, and they all have raised brass. But knowing the varietal approach that AF took to manufacturing in order to get trains out the door, your plates could very well be reversed!

I do have a nickel(?) 6 million plate that the colors are on top of the base metal.

We're probably both right!