ballast

Material place on a track bed to hold the track in line and elevation and to distribute its load. Suitable material consists of hard particles (e.g., crushed rock, slag, gravel) that are stable, easily tamped, permeable, and resistant to plant growth. (TRB1)

Heavy substances loaded by a vessel to improve stability, trimming, sea-keeping and to increase the immersion at the propeller. Sea water ballast is commonly' loaded in most vessels in ballast tanks, positioned in compartments right at the bottom and in some cases on the sides, called wing tanks. On a tanker, ballast is seawater that is taken into the cargo tanks to submerge the vessel to a proper trim.