: It is generally recommended to use "Fine" grade for most scales (even HO) to avoid an over-scale, "chunky" look.
: You can buy silica sand blasting grit or granite chick grit from agricultural supply stores for a fraction of the price of hobby-branded bags. 2. Heavy Equipment & Tractors (The "Safety" Buy)
: Professional modelers often recommend Arizona Rock & Mineral because it is real, scale-accurate crushed stone. Cheaper options like Woodland Scenics often use crushed walnut shells, which some users find can float when applying liquid adhesive or crack over time.
In the world of machinery, ballast is a safety requirement to prevent tipping during heavy lifting.
For hobbyists, ballast is the crushed rock that beds the track. Buying the right kind is a balance between scale accuracy and ease of use.