Abstract: Magnetic fields of electrical operating components of electric-traction-motor powered vehicles, especially diesel electric and all-electric railway locomotives, are sensed as the vehicles pass given locations along their respective paths of travel, and the signals so obtained are processed to yield perceptible outputs, such as oscillograph charts or computer printouts, indicative of performance and any maintenance work that must be done for maximum utilization of available energy. In instances in which speed of the vehicle is a significant factor, that information is ascertained and utilized in the processing of the signals derived from sensing the magnetic fields.