hopper wagons can only be unloaded by gravity with no external assistance and are therefore also classed as self-discharging wagons. the majority may be filled, when at rail or road level, by high-level discharge chutes (whose ends are more than 70 cm above the top of the rails) or conveyor belts. because a controlled amount of the load can be discharged at any place the wagons may be sent anywhere and are even used individually. railway companies also use hoppers as departmental wagons in maintenance of way trains for ballasting the track.