There are many ways to cover the screw holes, just let your imagination run. I have used very thin strip wood about the same width as the floor planks, painted them the color of the planks (or very close to it), and lightly glued them over the width of the flat car over the hole. They are hardly noticeable. You can add additional loads to what is already on the flatcar. Sandbags, barrels, crates, etc, could be added too. Gear workings off of defunct tools might work too. The fun will be to give that flatcar character of you own flair.
Rick
