I used an MPC gondola car and a magnet from a local hardware store. When I first built my layout, this picked up all kinds of "fuzzies". As for cleaning, I put a little piece of clear packing tape on the magnet. That seems to have little of no effect on the magnet. Peel the tape off and the metal drops off with it.
I concur that Lionel or MTH are missing the boat if they don't off one. It also picks up the assorted screw or detail part that may fall off while operating. My magnet straddles the center rail.