(c) 2006 Andrew Alden, licensed to About.com, Inc. (fair use policy)
The crushed-stone cobbles in this railroad track are known in the trade as road metal. The term goes back to the early 1800s, when the first railroads and macadamized roads were being laid down. The use of "metal" to mean any hard, tough material goes back two centuries further. Road metal still means today what it did then: crushed stone suitable for the most demanding applications.
Road beds and foundations use large amounts of crushed stone. Every outpost of civilization has a quarry somewhere near to supply this basic commodity. Often, road metal consists of traprockdark basalt or similar stone. In other places where that type of rock is scarce, industrial clinker or volcanic scoria are used instead. Wherever you live, this humble, basic substance has been dug, processed, sold and bought somewhere in your neighborhood. You can make a good living from crushed stone.