Abstract: A monitor to diagnose wheel assemblies of passing railroad trains, the monitor being characterized in that a cavity is present in and parallel to a measurement tie or in a hollow railroad tie, the cavity housing infrared deflection units and at least one infrared detector which make it possible to monitor all heat sources present in the vicinity of the wheel assembly and the undercarriage of the train car while offering maximum protection against mechanical and electrical factors. The wheel pressure and accelerator pickups introduced with the monitor allow accurate determination of the values needed to normalize the infrared test results such as wheel position, wheel weight and tread conditions.