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This paper presents a new stage of on-going research to fully characterize the rolling contact fatigue (RCF) damage observed in rail axle Compact Tapered roller Bearing Units (CTBUs). The bearings examined in this work have been removed from service, following the identification of degradation using on-board condition monitoring techniques. The running surfaces of the bearings were examined and the damage fully characterized using a mixture of metallographic, surface profilometry and high-resolution micro-Computed Tomography (μ-CT) techniques. In this manner the RCF was categorised by initiation mechanism: sub-surface or surface. This work has led to an increased understanding of the propagation of sub-surface and surface initiated RCF and its implications for life and condition of the bearing once failure has initiated. It was determined that the sub-surface initiated RCF damage was the first to occur in service and produced large craters on the bearing running surface. Once material had been lost in this primary manner, further secondary surface-initiating RCF grew from the edges of the large craters. It was deemed that the deep primary sub-surface RCF craters were more life limiting than the secondary surface damage. Therefore maps of the damage were created allowing the two mechanisms to be examined separately, both visually and by quantitative parameters such as volume loss, area, depth, roughness etc. [ABSTRACT FROM AUTHOR]