Ballast (Railway) : Material placed on a track bed to hold the track in line and elevation; it consists of hard particles that are stable, easily tamped, permeable and resistant to plant growth
Ilan Kelman (ed.), Vulnerability and Resilience Definitions and References, Version 2, 2007. http://www.islandvulnerability.org/docs/vulnres.pd f, (Vulnr and Resilience - Brit) {524}