Studs appear superficially very similar to squats: in particular they have two characteristic indentations in the rail surface. Although their cause is not yet well understood, cross-sections and circumstantial evidence suggest that they may initiate from thermal damage. They are associated mainly with areas of high traction demand and where friction differs on the two rails e.g. in curves rather than straight track. They do not occur in areas of sustained high friction, particularly in dry tunnels. Studs usually occur on only one rail, although they can appear, often with different severity, opposite one another (when it may be difficult to differentiate them from a wheelburn). Sometimes the stud initiates corrugation, as shown here. Multiple studs are common.
There is some association between studs and changes in rolling stock, particularly from DC to AC traction.
A cross section through a stud shows that there are sub-surface cracks, with a longer crack below the “running off” end of the crack mouth. These cracks cause the surface irregularities, with the larger irregularity above the longer crack. Although there are similarities with RCF, there are also significant differences.
Squats, shells, wheelburns and studs represent significant irregularities on the running surface of the rail, which result in high impact loading. They are also a source of transverse cracks, as a result of which ultrasonic inspection of the rail may be impossible. Consequently these defects should be removed, by localised grinding (which is possible only for the shallowest of defects), repair welding or rerailing. Routine grinding offers a viable preventative maintenance technique for most of these types of defects.