A test apparatus for testing an IR sensor of train undercarriage temperatures is disclosed. The IR sensor may be used to obtain infrared IR emission data by sensing a wheel or a wheel bearing of a rail vehicle. The test apparatus may comprise a heat emitter for supplying IR emissions at a reference temperature to the IR sensor. A support may support the heat emitter at a position spaced from the passage of the rail vehicle and in an orientation for directing the IR emissions at the IR sensor.