is badly fouled, the clogging will reduce its ability to drain properly. That, in turn, causes debris to be sucked up from the sub-ballast, causing more fouling. Therefore, keeping the ballast clean is essential. The principle cause of ballast fouling is "fines"—tiny powder-like particles abraded from the ballast material itself. Spillage of coal or other material onto the track can also contribute. 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 that job was done historically by manual labour, that process is now, as with 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, which washes the ballast and deposits the dirt and ballast into other wagons for disposal or re-use. Such machines can clean up to of ballast in an hour. Bioremediation can be used to clean ballast. 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, or where there are platforms. Where the track is laid over a swamp, such as the Hexham swamp in Australia, the ballast is likely to sink continuously, and needs to be topped up to maintain its line and level. After 150 years of topping up at Hexham, there appears to be 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