Kitty Litter ballast. This is a very nice "Meijer" store brand "Main Choice" unscented litter that comes in a 14lb jug. The price of this jug of "ballast" is around $5. A far cry from the $4 or more for a few ounces of "hobby" store ballast. $5 for 14 pounds of ballast. You will never need to buy ballast again. The lower track of the two has been ground down a little bit. The top is the litter right out of the jug. Slightly oversized for N scale. It is close enough for most, but if you insist on it being perfect size, you can take this white clay litter and crush it with a pestal in a bowl to make it as small and fine as you want..