I use a box car that has a lot of track clearance underneath. The reason for this, is:
I use a 1/4" thick piece of masonite w/the rough side down against the track, made
1/8 of & inch narrower than the box car on both sides. I attach the masonite w/ 3/16
machine screws thru the floor of the boxcar.(2) You have to make the screws going thru the masonite countersunk to clear any ballast that may be sticking up between the rails. The screws are attached in the center of the masonite. Some people put springs between the box car floor & the masonite. Not me. I just let the masonite
piece drag on the rails & keep the car directly behind my engine at all times.
About once a month clean off the dirt on the masonite w/a pc. of med. sandpaper.
My rails stay extremely clean. You can also run the box car in front of the engine to clean the rails for about a week when you first get started.
I also use metal polish to clean old rails before I start my track cleaner.
Larry