After one of our club members somehow sucked a steel machine screw off of the roadbed and into the armature of his open-frame-motered loco a few weeks ago (crackle-pop!), it became apparent that a magnetic track- cleaning car would be a good addition to the track-cleaning MOW rig that I run around the layout before we begin our operating sessions.
Experimentation proved that (A) they were *very* difficult to pry back off of a flat steel surface unless you scooted them over to an edge, where you could get a grip on them, (B) if you carelessly allowed a fold of your fingertip flesh to get between the magnet and said flat steel surface, you got a cute -and painful- little bloood-blister as a result, and (C) one of these magnets sits quite nicely atop an old Athearn flat car -held there securely by it's attracton to the sub- floor steel weight- and picks up anything ferrous that comes within circa 3" of the car as it passes.
No pics yet, but hang in there.
~Pete