d converter serves to convert the analog heat signal of fig2 into a series of discrete voltage values occuring at fixed time intervals dependent upon the sampling rate of a d conversion since the speed of the train will determine the length in time of the waveform of fig2 and since the scanning spot size determines the maximum number of useful samples that can be obtained the wheel speed information from detector 34 can be used to control the a d sampling rate to minimize the processing of redundant data in other words if the rate of a d sampling were not controlled a train travelling at 10 miles per hour would provide five times the samples of a train travelling at 50 miles per hour through the sensing zone the use of train speed to control a d sampling enables uniform a d spatial sampling conversions regardless of train speed the determination of the type of bearing under observation is made in a bearing discriminator circuit 40 such as that disclosed in the previously mentioned co pending and commonly assigned application ser no 135 628 filed mar 31 1980 for railroad car wheel bearing heat signal processing circuit it sufficies to say for the present application that the discriminator 40 determines whether the heat signals 22 are produced by roller bearings or friction bearings and its output is connected to roller and friction bearing processing circuits 42 and 44 respectively which determine if an overheated condition exists for the type of bearing.