Track ballast forms the trackbed upon which railroad ties (sleepers) are laid. It is packed between, below, and around the ties. 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 trains roll by. It is typically made of crushed stone, although ballast has sometimes consisted of other, less suitable materials, for example burnt clay. 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 (6 inches) thick; and high-speed railway lines may require ballast up to 1⁄2 metre (20 inches) 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 (12 inches) thick can lead to vibrations that damage nearby structures. However, increasing the depth beyond 300 mm (12 inches) adds no extra benefit in reducing vibration.
In turn, track ballast typically rests on a layer of small crushed stones: the sub-ballast. The sub-ballast layer gives a solid support for the top ballast, and reduces the seepage of water from the underlying ground. Sometimes an elastic mat is placed on the layer of sub-ballast and beneath the ballast, thereby significantly reducing vibration.
It is essential for ballast to be piled as high as the ties, and for a substantial "shoulder" to be placed at their ends; the latter being especially important, since this ballast shoulder is the main restraint of lateral movement of the track. The ballast shoulder always should be at least 150 mm (6 inches) wide, and may be as wide as 450 mm (18 inches).
The shape of the ballast is also important. Stones must be irregularly cut, with sharp edges, so that they properly interlock and grip the ties in order to fully secure them against movement; spherical stones cannot do this. 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; this, in turn, causes more debris to be sucked up from the sub-ballast, causing more fouling. Therefore, keeping the ballast clean is essential. Bioremediation can be used to clean ballast.
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, if shoulder ballast is removed to the correct depth. While this job was historically done by manual labour, this process is now, like many other railway maintenance tasks, a mechanised one, with a chain of specially-designed railroad cars handling the task. One wagon cuts the ballast and passes it via a conveyor belt to a cleaning machine, then the cleaning wagon washes the ballast, and deposits the dirt and ballast into other wagons for disposal and re-use, respectively. Such machines can clean up to two kilometres (1.2 mi) of ballast in an hour.
Cleaning, however, can only be done a certain number of times before the ballast is damaged to the point that it cannot be re-used; furthermore, track ballast that is completely fouled can not be corrected by shoulder cleaning. In such cases, it is necessary to replace the ballast altogether. One method of "replacing" ballast, if necessity demands, is to simply dump fresh ballast on the track, jack the whole track on top of it, and then tamp it down; alternatively, the ballast underneath the track can be removed with an undercutter, which does not require removing or lifting the track.
The dump and jack method cannot 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, and needs to be topped up to maintain its line and level. After 150 years of topping up, there appears to be 10 m (33 ft) 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. The shoulder acquires some amount of stability over time, being compacted by traffic; maintenance tasks such as replacing ties, tamping, and ballast cleaning can upset this stability. After performing these tasks, it is necessary for either trains to run at reduced speed on the repaired routes, or to employ machinery to compact the shoulder again.
If the trackbed becomes uneven, it is necessary to pack ballast underneath sunk ties to level the track out again. This is, in the mechanized age, usually done by a ballast tamping machine. A more recent, and probably better, technique is to lift the rails and ties, and to force stones, smaller than the track ballast particles and all of the same size, into the gap. This has the advantage of not disturbing the well-compacted ballast on the trackbed, as tamping is likely to do. This technique is called pneumatic ballast injection (PBI), or, less formally, "stoneblowing". However, this technique is not as effective with fresh ballast, as the smaller stones tend to move down between the larger pieces of ballast.
The quantity of ballast tends to vary with gauge, with the wider gauges tending to have wider formations. The depth of ballast also tends to vary with the density of traffic, as faster and heavier traffic requires greater stability. The quantity of ballast also tends to increase over the years as more and more ballast is piled on. Some figures from an 1897 report are: