Here's something you can do with one of those Allis-Chalmer frames without the load that nearly everybody has knocking about the collection or that you have sadly passed by at the train show...
Thanks to @trainroomgary for the original inspiration! 
As you can see, a stock Magic Eraser fits just fine under the car...
The trick, of course, is to hold it in place while cleaning. Despite their name, Mercedes 600 Pullmans didn't really do the job...
The answer lies in an obscure bit of floral technology called the "Flower Frog". Why it's called that, I dunno...
This particular one came from eBay.
Popsicle sticks (sold in a bag as 'craft sticks' if you're not allowed sweets) make excellent cross supports. Here, I've laminated three together with wood glue and am cementing the cross brace into place with JB Weld...
The finished assembly. A tad off center, but, then, so am I. 
Marked for drilling.
And assembled with 4-40 screws.
An eraser installed and ready to test!
Alcohol soaked wet test run with the Wabash Cannonball!
Even though this track was cleaned a few days ago, we're still picking up a fair amount of crud!
Test 2 with some D cell battery ballast...
Got even more schmutz!
Atsa shiny track, eh, boss?
We conclude with some test footage of the Allis-Chalmers Track Cleaner in action...
Hope this helps! If you build one, let me know how it works for you!
Mitch
