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Asynchronous induction motors are actually the most used in rail vehicle traction chains. Although more reliable and robust than dc motors, they are vulnerable to insulation failures, particularly in dirty environments. The dirt accumulated inside motors is one the main causes of insulation failure. Excessive accumulation of dirt blocks in ventilation vents and motor air gap may lead to overheating and, in severe cases, abrasion of the insulation. Taking advantage of the peculiarities of environmental rail dirt (dirt with a high concentration of ferromagnetic particles), a new predictive technique has been developed which is able to detect the presence of excessive motor dirt in operating conditions. This new technique to determine the degree of contamination in the motor air gap is based on the measurement of magnetic leakage flux and its subsequent analysis in the frequency domain. The results of the technique make it possible to detect dirt accumulations early, before breakdown occurs and without the need to stop the machine.