compacted by traffic, but maintenance tasks such as replacing ties, tamping, and ballast cleaning can upset that stability. After performing those tasks, it is necessary either for trains to run at reduced speed on the repaired sections, or to employ machinery to compact the shoulder again. If the trackbed becomes uneven, it is necessary to pack ballast underneath sunken ties to level the track again, which is 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. That has the advantage of not disturbing the well-compacted ballast on the trackbed, which tamping is likely to do. The technique is called pneumatic ballast injection (PBI), or, less formally, "stoneblowing". However, it is not as effective with fresh ballast, because the smaller stones tend to move down between the larger pieces of ballast. Quantities The quantity of ballast used tends to vary with gauge, with the wider gauges tending to have wider formations, although one report states that for a given load and speed, narrowing the gauge only slightly reduces the quantity of earthwork and ballast needed. The depth of ballast also tends to vary with the density of rail 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 onto an existing roadbed. Some figures from an 1897 report listing requirements for light railways (usually narrower than standard gauge) are: first class line – rail – . second class line – rail – . third class line – rail – . See also Ballastless track Ballast tamper Gandy dancer Maintenance of way Track maintenance Footnotes References International Federation for Structural Concrete (fédération internationale du béton) bulletin #37. Further reading New South Wales: Ballast 1850-1987 Longworth, Jim Australian Railway History, December, 2004 pp443–462 External links Photos of ballast cleaners in the UK Photos of ballast regulators in the UK Permanent way Rail infrastructure