require ballast up to 0.5 m 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 confers no extra benefit in reducing vibration.

In turn, track ballast typically rests on a layer of sub-ballast, small crushed stones which provide a solid support for the