This paper studies the influence of an energy storage system (ESS) on the fuel consumption of a diesel-electric locomotive. First, an energetic model of a diesel-electric locomotive is established using energetic macroscopic representation (EMR). An inversion-based control is deduced, and the model is validated by experimental results on a real locomotive. Second, from this validated model, a battery/supercapacitor ESS is added in simulation to study the benefit of hybridization before integration on the real vehicle. The simulations show that simple energy management based on a frequency approach allows for the reduction of 25% on fuel consumption on a real drive cycle.