Abstract: A mobile device for testing an axle-mounted roller bearing of a railcar to determine possible defects in the bearing. A movable base plate has an electric motor, a drive roller, a driven roller, and a plurality of leveling jacks mounted thereon. The motor actuates the driver roller and the leveling jacks raise the base plate to move the drive roller and driven roller into contacting engagement with the outer race of the roller bearing. The outer race of the bearing is rotated by the drive roller at a predetermined rate. Shock pulses of energy also known as an acoustic emission, are emitted by defects in the bearing. A probe which senses such emissions is connected by a cable to a monitoring instrument. The probe is placed on a test point having direct metallic contact with the bearing, whereby the monitoring instrument will measure the type and magnitude of shock pulses, if any, emanating from the bearing. These readings are used to determine the type and extent of possible defects present in the bearing.