An energy storage system for connection to a traction power supply that provides power to an electric vehicle. The energy storage system includes a power controller that controls a DC-DC converter to transfer electrical energy from the traction power supply to electrical energy storage when the train is braking. The power controller also controls the converter to transfer electrical energy from the electrical energy storage to the traction power supply when the train is accelerating. The controller slows the rate of energy transfer when upper and lower voltage boundary limits of the electrical energy storage are approached, respectively.