Abstract: Wheel testing apparatus according to the invention comprises at least one search unit positioned "in-track" replacing a portion of a rail so that the gauged wheels of a passing railroad train roll thereover. Each search unit has a thin vertical rail aligned with the gauge edge of the track for supporting each passing wheel adjacent to its flange, and thereby exposing the running surface of the wheel for testing in a zone which does not interfere with the wheel's path.Each search unit has two ultrasonic transducers enclosed in a flexible fluid-filled boot and are positioned adjacent to the thin vertical rail in the test zone under the running surfaces of passing wheels, the fluid-filled boot providing good ultrasonic coupling therewith.The transducers are angled and opposed to send ultrasonic pulses around the wheel in opposite directions. Echoes indicate defects and through transmission times indicate wheel size and wear.