The thing that makes flatcars challenging is surely how to make them heavy enough to stay on the track when they are empty. If it makes you feel any better, it looks like real railroads had the same problem back in the wood car era. They appear to have blocked empty flats near the back of the train for this reason.
My friend Brian Pate made his many flat cars with lead decks, which he scribed to represent individual boards. Some years ago, I went and purchased enough 1/32″ lead sheet to outfit a large roster of flat cars in this way. Maybe I should still take this approach.
My current preference is to hide the weight as a false floor above shallow stringers. I think Jack Burgess suggested the use of printed paper to disguise the weight. The question is, how much lead do we need?
The NMRA preaches one ounce plus half ounce per inch of length. These 33-foot cars will be just over four inches long, and so, they should be at least three ounces. To get a sense of the size of such a piece of lead, I drew up the flat car in OnShape and weighed it. Two layers of my 1/32 sheet will come out a little light at 2.3 ounces.
I investigated tungsten as well, and this comes out at a comfortable 3.5 ounces, assuming 1/16″ sheet. There is an easy online source for tungsten at tungsten.com. They will even cut the material to your specifications, which is well worth it because the material is about as easy to work with as an American president. I found their service exemplary, and received a quote the day after uploading my drawing. Unfortunately, it came out at over $30 per car; that’s a little rich for me, but it may be okay for others.
I really like this facility to predict the mass of a model in the drawing stage. It’s yet another reason to use OnShape.