In the mid-2000s, a new rail defect that was initially classified as a squat became increasingly common on London Underground's track. By 2006, there were about 600 of these and they had become the Underground’s single most common rail defect. This defect occurred almost exclusively on lines carrying relatively new rolling stock. The work reported here was undertaken initially to characterize this defect, advise as to whether it was indeed a squat, and propose a hypothesis that explained its mechanism of formation. This paper includes observations and track measurements made in the field and initial results of a metallurgical analysis. The hypothesis for formation of the defects is presented, and both similarities and differences are discussed between these defects and the classical ‘squat’. The defect on London Underground appears to be the same as that described by Marich and his colleagues in Australia and by Li and his colleagues in the Netherlands. It is evidently not a conventional rolling contact fatigue defect. In order to avoid confusion arising from simple misuse of an established term, it is proposed that these defects be given a different name, for which ‘stud’ is proposed. Evidence to date is that the ‘stud' is a significantly more benign defect than the ‘squat’.