To establish a single European railway area, the European Commission requires, by 2019, that energy billings shall be computed on the actual energy consumed. So, in the near future, as required by EN 50463-2, all the trains shall be equipped with an Energy Measurement Function (EMF), whose measurement accuracy should be assessed and periodically re-verified. To answer to this emerging need, this paper presents a calibration system allowing the accurate testing of EMFs in DC conditions, up to 10 kV and 300 A, with arbitrary superimposed spectral component up to 10 kHz. It is able to reproduce all the tests prescribed by EN 504632, but, in order to characterize the EMF in actual operating conditions, also a number of possible synthetic complex waveforms and test signals acquired on board trains can be reproduced. Some testing results of an EMF, which uses voltage and current transducers typically installed in Italian electro-trains, are presented.