A plurality of sensors, each generating distinct signals in response to the passage of leading and trailing edges of car wheels, are positioned along the rail at a height above the rail to detect the passage of a selected chord on the wheel tread. The distances between initial sensors and the final unit equal the various selected chord lengths on conventional sized wheels. When one of the initial sensors detects the passage of the trailing edge of the wheel at the chord level simultaneously with the detection of the leading edge of that wheel by the last unit, the output signals are applied to a logic circuit, which registers a wheel of corresponding diameter. Additional sensors placed immediately adjacent the final sensor provide measurement of actual wheel diameters slightly less than the standard sizes due to wheel wear. When the sensors detect a gap in the tread chord, as a worn wheel passes, greater than a predetermined length for each diameter wheel, an indication is provided of a worn wheel requiring replacement. A second set of sensors positioned on the other side of the rail measures the flange height to also indicate wheel wear. All measurements are determined by logic elements and supplied to the yard control system. The diameter and tread thickness measurements are used to supply a corresponding wheel rotational inertia factor to the speed control system while tread and flange wear indications indicate cars requiring immediate wheel replacements.