Track ballast

[1] It is used to bear the compression load of the railroad ties, rails, and rolling stock; to facilitate drainage; and keep down vegetation that can compromise the integrity of the combined track structure.

[1] 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.

[8] It is essential for ballast to both cover the ties and form a substantial "shoulder"[5] to restrain lateral movement of the track.

Speed limits are often reduced for a period of time on sections of track where fresh ballast has been laid in order to allow it to properly settle.

Where the track is laid over a swamp the ballast is likely to sink continuously, and needs to be "topped up" to maintain its line and level.

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.

A more recent, and probably better,[6] 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.