In the railroad industry, bearing hot-box detectors are the primary tools utilized for the removal of distressed bearings from service. Current technology has expanded the role of these detectors to monitor bearings that appear to "warm trend" relative to the average temperatures of the remainder of bearings on the train. Several bearings set-out for trending and classified as non-verified revealed that a common feature was discoloration of rollers within a cone assembly. A number of laboratory tests were performed to determine a minimum temperature and environment needed to reproduce these discolorations. In addition, a metallurgical examination concluded that microhardness profiles from discolored rollers exhibited evidence of heating, which lead to softening of the case microstructure, and a reduction in case depth. In order to determine a cause for the internal heat leading to the discoloration of rollers, selected laboratory experiments were carried out in a simulated service environment. Testing focused on exploring certain defects or hypothetical scenarios that may raise the bearing cup temperature above that of normal operating conditions. None of the examined cases resulted in similar roller discolorations to those observed in the trended set-outs. The studies presented in this paper comprise the initial work aimed at determining the root cause of warm bearing trending that some tapered roller bearings experience during service.