I use a Lionel Boxcar with a flat metal bottom. Place 2 magnets on the bottom of the boxcar.
These magnets are available at an Ace Hardware Store. I use Great Lake Ace Hardware in Commerce Twp., Michigan. I place this boxcar behind my North East Track Cleaning car and it does pick up junk from the track. I use a LionChief Plus, Diesel locomotive. FT • A-B-A, all these locos have power.
Side view
Bottom view
Hope this helps: Gary