This paper describes the measurement system used to monitor the energy dissipated in the braking rheostat resistors on board a locomotive operating in a DC rail system. The aim of the activity is the accurate estimation of the energy, nowadays wasted, that could be recovered thanks to the implementation of the smart grid paradigm in the DC railway system. To provide metrological reliability to the measured wasted energy, a preliminary identification and estimation of the uncertainty is provided. The target uncertainty of the measurements is 1 %.