Track ballast forms the trackbed upon which railroad ties are laid. It is packed between, below, and around the ties and it is used to bear the load from the railroad ties, to facilitate drainage of water, and also to keep down vegetation that might interfere with the track structure. This also serves to hold the track in place as the roll by. It is typically made of crushed stone, although ballast has sometimes consisted of other, less suitable materials, the term ballast comes from a nautical term for the stones used to stabilize a ship. The appropriate thickness of a layer of track ballast depends on the size and spacing of the ties, the amount of traffic on the line, and various other factors. Track ballast should never be laid down less than 150 mm thick, an insufficient depth of ballast causes overloading of the underlying soil, and in unfavourable conditions overloading the soil causes the track to sink, usually unevenly. Ballast less than 300 mm thick can lead to vibrations that damage nearby structures, however, increasing the depth beyond 300 mm adds no extra benefit in reducing vibration. In turn, track ballast typically rests on a layer of crushed stones. The sub-ballast layer gives a solid support for the top ballast, sometimes an elastic mat is placed on the layer of sub-ballast and beneath the ballast, thereby significantly reducing vibration. The ballast shoulder always should be at least 150 mm wide, the shape of the ballast is also important. Stones must be cut, with sharp edges, so that they properly interlock and grip the ties in order to fully secure them against movement. In order to let the stones fully settle and interlock, speed limits are often lowered on sections of track for a period of time after new ballast has been laid. If ballast is badly fouled, the clogging will reduce its ability to drain properly, therefore, keeping the ballast clean is essential. Bioremediation can be used to clean ballast and it is not always necessary to replace the ballast if it is fouled, nor must all the ballast be removed if it is to be cleaned. Removing and cleaning the ballast from the shoulder is often sufficient, such machines can clean up to two kilometres of ballast in an hour. In such cases, it is necessary to replace the ballast altogether, the dump and jack method cannot of course be used through tunnels, under overbridges, and where there are platforms. Where the track is laid over a swamp, such as the Hexham swamp in Australia, the ballast continuously sinks, after 150 years of topping up, there appears to be 10 m of sunken ballast under the tracks. Chat Moss in the United Kingdom is similar, regular inspection of the ballast shoulder is important, as noted earlier, the lateral stability of the track depends upon the shoulder
Good quality track ballast is made of crushed stone. The sharp edges help the particles interlock with each other.
Track ballast (close up) between railway sleepers and under railway track
New track ballast is placed at the Boxmeer railway station, The Netherlands.