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The braking energy in diesel-electric locomotives is typically wasted into resistors. A more energy-efficient way is to store and recycle such energy. Thus, this article proposes a multiport power conversion system as the core of a hybrid energy storage system, based on Lithium-ion (Li-ion) batteries and supercapacitors (SCs). In such configuration, SCs act as a buffer against large magnitudes and rapid fluctuations in power, thus reducing current stresses in the battery system. Unlike other alternatives, this solution uses the recovered energy to supply only auxiliary loads, while the diesel generator keeps powering traction motors. As a consequence, the required energy storage capacity for the system is considerably reduced, which reflects reduced cost and volume. Further, the proposal can be used to retrofit existing systems and can also be adapted for other types of locomotives. To validate the proposed concept, a 7.6-kW scaled-down prototype was implemented using electric motors to emulate the locomotive power-train structure. Experimental results demonstrate the effectiveness of the power management strategy under different operating scenarios. [ABSTRACT FROM AUTHOR]