The credit for this design goes to another forum member whose name has gotten away from me, but hopefully he will come to the forefront when he sees this post. My design philosophy is KIS (Keep It Simple), and the track cleaning car in the sketch defines KIS in a superlative way. Not only is the car simple to build, it is very inexpensive and it works great!
I have built and tested this design using an MTH Rail King gondola that comes with a heavy junk load, and a Lionel 6424 gondola that does not come with a load. A double layer of glass marbles increases the weight of the Lionel gondola just the right amount for operation on my layout having over 300 feet of mainline MTH Real Trax and 14 mainline switches.
The Scrub-It Scouring Pad, as bought, measures 3.75" x 2.75" x 1.25", so some trimming is necessary to make it fit. Use a box knife with a long extendable blade to do the the trimming.
The scouring pad material is like a 3M Scotchbrite pad, and it works well without any track cleaning fluid. Running at a scale 25 MPH, no significant snagging was observed. That includes running through switches, grade crossings, and track joint gaps. Nothing is perfect, and this design is no exception, but the only negative I found is that the Scrub-It pad on the longer wheel base Rail King gondola barely scrubs the outside rail on 042 curves. The shorter wheel base Lionel 6424 gondola works better on sharp curves.
As for costs, this is what I spent to build both cars:
Rail King gondola $35
Lionel gondola $5
Adhesive pads, 24 for $1
Scrub-It pads 6 for $1.
So excluding the cost of the gondolas, they cost 25 cents each to build!
To copy the sketch, go below and click on its icon. When the sketch opens, right click on it and do a "Save Image As" to your desktop. Then move the file anywhere you want to keep it.