1. Her er en beskrivelse av "head checks": " "The stress on rails has constantly increased during the last few years in-particular due to high train speeds and heavy axle loads. For this reason the relevance of near-surface defects in rails has been gaining in significance. The cause of such defects is usually rolling contact fatigue. *One such defect is the so-called head check. Head check cracks occur at the gauge corner of the rail – mainly on the outside rail of a curve.*
KILDE: http://www.ndt.net/article/ecndt2006/doc/P178.pdf

2. Her er en definisjon av "head checks": "THE TERM "head checks" is used to refer to a type of cracking in rails that occurs particularly on curves (Figure 1). As a result of excessive localised plastic deformation of the rail head, which occurs in response to increasing shear forces and decreasing contact area between the wheel and the rail, small cracks appear in the contact area. In time, these cracks propagate further in the rail head."
KILDE: http://findarticles.com/p/articles/mi_m0BQQ/is_6_39/ai_55492...

3. Her er en illustrasjon av "the gauge corner crack" (se punkt 1): http://www.ndt.net/article/v07n06/khalid/khalid.htm
(Søk på "the gauge corner crack" for å finne illustrasjonen").

4. Her er en definisjon av "squat": "In those areas where the greatest crack depths were determined ultrasonically, dark-coloured markings were commonly visible on the running surface (photo 2). This indicates hammering of the running surface during the passage of wheels as a result of a local drop of the running surface caused by an underlying network of cracks. This type of damage is often termed "squat."
KILDE: http://findarticles.com/p/articles/mi_m0BQQ/is_6_39/ai_55492...