For several years, the Association of American Railroads (AAR) has been developing new techniques to detect defective roller bearings as part of their new generation wayside acoustic detector program. This paper reviews thermal and vibration data collected from on-board a test train used to simulate railroad revenue service conditions during the test program. The train tests were carried out by Transportation Technology Center now known as Transportation Technology Center, Inc. (TTCI), a subsidiary of the AAR, at the Transportation Technology Center (TTC), Pueblo, Colorado in November 1996. Of all the bearing defect types to be detected, one of the most challenging is that of a bearing with a loose inner raceway commonly referred to as a spun cone. Normal roller bearings have "press fit" inner raceways that keep them from rotating or sliding about the axle. However, the spun cone bearing has lost its tight press fit and can slowly rotate about the axle journal axis. The spun cone bearing defect is suspected to be the cause of many of today's confirmed hot bearing setouts. This paper compares both thermal and vibration data from bearings with no internal defects to those with spun cones, broken rollers and water etched surfaces.