Steel, indeed most metal, "flows" (don't know the technical word for it) under pressure. As enough wheels apply pressure on the inside of the top and inside of the rail, some metal is being slowly moved to the outside edge, and then flaking off. I saw this on the B&M main in New Hampshire in the late 1970s - but I wouldn't consider the B&M of that time well maintained.
So, it's normal wear, but probably overdue for maintenance as a previous post suggests.