I think most people these days that use an in-train method of cleaning the rails use some variety of commercial unit such as those made by Centerline Products and the like. That's not to say the Masonite pad under the boxcar isn't still popular but I have to admit I recall only ever seeing someone use that once in my lifetime.
For myself, if I ever find the need to have a rail cleaning vehicle I think I'd make a work train style set up, something like a rail grinder or weed sprayer train that has the sliding pads under it. The pads can be easily disguised with such a train. You can let your imagination run wild on the design and it doesn't even have to be big, Hamerlsey Iron, one of the top heavy haul railways in the world used this diminutive rail grinder train for a few decades before replacing it with a much larger unit.
https://www.westonlangford.com/images/photo/111916/If you can find one Model Die Casting/Roundhouse many eons ago made their boxcab diesel model with a track cleaning setup. It used sandpaper pads which I would not recommend using but I'm sure something else could be substituted.
- Tim
