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 discusses data collected from several bearings with spun cone defects under both laboratory and simulated revenue service environments. The AAR performed the laboratory tests in July 1996 and the simulated revenue service test was completed in November 1996. Of all bearing defect types to be detected, the most challenging and highest priority is that of a bearing with a loose inner raceway (spinning cone) which rotates about the axle mount. Normal roller bearings have “press fit” inner raceways which keep them from rotating or sliding about the axle. The “Spun Cone” bearing defect is responsible for many of today’s bearing related derailments.