I. Introduction
The transportation safety and efficiency of freight wagons can be significantly improved by equipping them with on board sensors for continuous monitoring and tracking. The lack of electricity in freight cars, needed for powering on board sensors, can be overcome by resorting to the harvesting of otherwise wasted energy [1]–[3]. As a significant amount of energy is usually dissipated in railcar suspensions through oil shock absorbers or frictional dampers, a large variety of train suspension energy harvesters (TSEHs) have been proposed in the literature [3]. Examples are linear electromagnetic harvesters, where magnets move linearly with the train suspension inside coils [4], [5], or rotary electromagnetic harvesters, which are able to convert the linear motion of the suspension into a bidirectional [6] or a unidirectional rotation [7] of an electromagnetic generator. The last ones are based on mechanical motion rectifier (MMR) systems that increase the efficiency by allowing the generator to always rotate in the same direction [7]–[17].