I’ve spent the past week experimenting with this very thing. A while back someone asked about this in this thread, and I experimented with printing a concrete texture onto paper and inserting that between the rails. It could go the full width of the rails without interfering with the flanges.
Since then I’ve wanted to improve upon that and have been playing with depron foam and craft (EVA) foam. The stuff you get at Michael’s and Hobby Lobby is too thick for code 100 rail, but 1mm thick foam should work. I couldn’t find any 1mm thick foam locally so I’ve had to order some from an ebay seller. I used black foam with gray paint sponged on for asphalt, and white foam with spray paint speckled on for concrete. When the 1mm foam arrives I’ll try it out and report back.
I’ll post a couple of pictures of some of the early tests I did with the thicker foam.
Steve S