## 3 Results
### Hybridization Possibilities  
Hybrid units can be new, but sometimes it seems reasonable to rebuild \"middle-aged\" units (<20 years) or units with a low number of kilometers (<2 million), which can increase their operational life and reduce capital costs of new units [41]. Fighting against programmed obsolescence is a challenge for many operators, such as the French operator SNCF [42], to reduce capital costs and support environmental concerns.  
As previously mentioned, Spanish operators typically use two locomotive series for the selected track: the Series 319.2 and Series 333.3 diesel-electric locomotives, with rated powers of 1450 kW and 2240 kW, respectively (both based on EMD locomotive platforms). As the authors have discussed in previous works [43], the average power output of these units is approximately 800-1300 kW, meaning that the full rated power is not applied if a storage device can handle peak demand periods.  
However, implementing freight hybrid trains presents challenges, particularly regarding the space needed to accommodate OESD. One potential solution is the use of a dedicated Battery-Electric Locomotive (BEL) alongside diesel units operating in a single consist, an approach tested by U.S. operators, e.g., [13]. This seems practical given the heavy weight of American freight trains, which often exceed 2500 tons. A second option is the use of a service vehicle, which has proven effective for passenger trains [44] but is more challenging for freight operations since a dedicated wagon for each train would be costly and complex because locomotives change from head to rear at CT and MU stations.  
Finally, the third option involves finding space for OESD integration within a conventional locomotive unit. This last approach has been considered in the present study. Reasons for this choice are discussed in the following paragraphs.  
At present, the main Spanish railway operator has decreased its freight activity [6; 45], and some new electric locomotives manufactured by Bombardier in 2010-2015 (units belonging to the TRAXX family) are surplus. This represents that older series 269.2 (built in 1985, designed by Mitsubishi Electric Co. [46]) are available for location (12 + 20 units of another 269 subseries). Moreover, around 18 units of the 319.2 and 333.3 series are still available for renting [47].  
Class S269 weighs 88 tons (its subseries are S269.2 and S269.6). They are 3 kV DC locomotives that develop 2 \(\times\) 1550 kW with two motor groups (mono-motor group in each bogie) with independent control and electronics (1-3 and 2-4 in Figure 13). The hypothesis is that one of these mono-motor groups and the 380 V-AC motor-generator group (covered by power electronics in modern diesel units [48]) can be replaced by OESD. The result is a locomotive with 1550 kW comparable to a diesel-electric unit series 319.2. As stated before, this reconstruction procedure is not new and has been applied by some manufacturers in DMU and EMU to convert them into hybrid EMUs [49]. Sometimes, hybrid units such as the Talgo S-730 Diesel Hybrid Electric Multiple Unit (DHEMU) have removed one of the two diesel generators located in a technical car to include a hydrogen fuel cell for tests of a research project dealing with green hydrogen [44]. It is important to note that a new active cooling system is necessary to control the temperature of OESD (15-20 \({}^{\circ}\)C in the case of batteries). Figure 13 shows the layout and dimensions of standard class S269.  
Tractive effort curves of diesel-electric and electric locomotives are depicted in Figure 14 and elaborated from public manufacturers' data. Figure 14b shows the joint tractive effort of the hybrid multiple units. For the sake of comparison, the tractive effort of a modern TRAXX series is shown in Figure 14b. Notice that TRAXX develops more power, but hybrid units develop high tractive effort at low speeds. All curves have been used in the simulation model for railway units (PBLM-Rail).