I wouldn't recommend using real salt, as it is corrosive. Encapsulated in paint, or glue might eliminate its corrosiveness, but I would recommend using crushed marble. It can be found at building supply houses that sell brick, stone and other masonry supplies. Crushed marble is used as an alternative to white sand, and looks very similar to real rock salt. If you only want to make a pile, you may be able to get a baggie full for nothing. If you want to make loads for hoppers, you can get a plastic pail for around 5 bucks. I've attached some pics of a salt load in a Lionel Post War 2 bay hopper.
