## 4 Conclusions  
This paper explores an alternative solution for the hybridization of freight locomotives operating on low-traffic routes, aiming to maintain their hauling capabilities while capitalizing on regenerative braking. The central hypothesis involves refurbishing mid-life conventional locomotives to extend their operational lifetime, an idea already proposed by some manufacturers for diesel-electric units. To achieve these objectives, an on-board energy storage device comprising electric double-layer capacitors and batteries is proposed. While EDLC technology has been applied to tramways, it remains largely unexplored for freight locomotives, with only a few battery-electric locomotives currently in operation. \begin{table}
\begin{tabular}{l c c c} \hline \hline \multicolumn{1}{c}{**Indicator**} & **EDLC** & \multicolumn{1}{c}{**Locomotive**} & \multicolumn{1}{c}{**Locomotive**} \\  & & **S269 (Hybridized)** & **Diesel S33** \\ \hline Max Power & 1570 kW & 1450 kW & 2200 kW \\ \(\Delta\) Energy consumption (fuel to electricity) & N/A & 150.9 kWh & \(-\)92.98 kWh 1  \\ Energy Recovered from resistive braking & 5.45\% & 25.33\% & 11.36\% \\ SoC range (max/min) & 1.05/0.24 & N/A & N/A \\ \(\Delta\) Fuel demand/day & N/A & - & \(-\)349 L 2  \\ \(\Delta\) Electricity demand/day 3  & 140–160 kWh & 220–250 kWh & - \\ Service in km/day 4 (km/year) & - & 260 (78000) & 260 (78000) \\ \(\Delta\) Energy costs 4 (year) & - & 6.48 k€ & 251.8 k€ \\ \hline \hline \end{tabular}
\end{table} 
Table 11: Main results from simulation (route Cartagena–Murcia). Figure 21: Results for the proposed management of OESD through PBLM: (**a**) EDLC voltage in p.u. ; (**b**) EDLC SoC; (**c**) Currents in the EDLC; (**d**) Revisited power of EDLC. To evaluate the feasibility of this approach, a simple, accurate, and physically based EDLC model was developed and tested in the laboratory. The results demonstrate that the model achieves an acceptable level of accuracy in capturing voltage and power fluctuations associated with EDLC charge and discharge cycles. Using these models, the paper presents a practical example of OESD sizing for energy management in a hybrid locomotive (diesel-electric and hybrid-electric) hauling a 1300-ton freight train. The proposed methodology highlights the importance of integrating physical models of railway units with EDLC dynamics, offering a preliminary framework for validating hybrid locomotive performance in real-world scenarios. The sizing process incorporates the latest advancements in EDLC and battery technologies, ensuring that the proposed OESD specifications are both credible and achievable. In conclusion, hybridizing locomotives with EDLC and batteries enables the recovery of approximately 40-50% of braking energy on routes with gradients of 1.0-1.2% while also reducing fuel consumption--and thus greenhouse gas emissions--by around 30%. This reduction could serve as a catalyst for shifting freight transport from road to rail. Additionally, on electrified tracks, hybrid units can act as mobile generation devices, supplying power to electric trains or restoring service during Overhead Line Equipment (OLE) failures. Future work will focus on developing detailed physical models for batteries and refining the OESD design through additional simulations and modeling of storage devices. Conceptualization: A.G. ([PERSON]) and M.C.R.-A. ; methodology: A.G. ([PERSON]), F.M. and M.C.R.-A. ; software: A.G. ([PERSON]) and A.G. ([PERSON]); validation: A.G. ([PERSON]) and A.G. ([PERSON]); formal analysis: F.M. and M.C.R.-A. ; investigation: all authors; resources: M.C.R.-A. and A.G. ([PERSON]); writing--original draft preparation: all authors; writing--review and editing: A.G. ([PERSON]) and M.C.R.-A. ; visualization: A.G. ([PERSON]); supervision: [PERSON]. ([PERSON]) and M.C.R.-A. ; project administration: [PERSON]. ([PERSON]) and M.C.R.-A. ; funding acquisition: A.G. ([PERSON]) and M.C.R.-[PERSON]. All authors have read and agreed to the published version of the manuscript.