Squats are RCF defects that occur in straight or gently curved track. They occur within the running band on the rail crown where the rail is heavily sheared from locomotive traction. Squats are sometimes known as “black spots”, where the dark area is a depression in the rail above the crack.
Squats are thought to initiate where small bits of hard material are trapped at the wheel/rail interface and cause one crack to grow rather than another. The direction of the crack can be felt, as with other types of RCF, by drawing a finger gently along the rail: the surface feels smooth when running a finger in the direction of traction but “ragged” in the opposite direction. The crack develops in the direction of the heavily sheared surface layer.
Essentially the same mechanism causes development of squats and GCC (Section 1.2). Transverse defects often result from squats that are left in track. Squats remain superficial in the absence of water.