Freight wagons typically do not have an electrical train supply and state-of-the-art auxiliary energy generation systems as, e.g. axle generators on freight wagons lack the possibility for a cost-effective retrofit. However, emerging applications, as, e.g. anti-lock braking systems for freight trains or condition monitoring systems for the loaded goods, require electric power in the watt range. Therefore, a non-invasive auxiliary energy supply system with 22 W/dm³ (360 mW/in³) based on a non-coaxial eddy-current coupling is described and analysed in this study. It is a kinetic energy recovery system, which can extract power directly from the motion of a freight wagon's wheel. An air gap of 10 mm between the wheel and a permanent magnet rotor is set during operation, making the system a non-invasive one. A model for the transfer efficiency of the system is established and verified by measurements. The prototype including an optimised generator and an integrated active rectifier for generating a DC-output voltage is built and an electric power of 8 W can be extracted from a steel wheel moving with a surface speed of 22 m/s.