## 1 Introduction  
Decarbonization is a key political objective for the coming decades. The transportation sector, accounting for a significant share of greenhouse gas (GHG) emissions (approximately 25% in the European Union during this decade), has become a primary target for electrification efforts. Within the railway sector, achieving electrification necessitates the complete electrification of railway infrastructure. However, this process is far from complete and can be described as progressing rather slowly. Table 1, built from data available in [1, 2], illustrates the percentage of non-electrified railway lines in the EU-27 overall, as well as in selected countries under varying scenarios, highlighting the changes in electrification rates from 2013 to 2022. While it is true that approximately 55% of the rail network in Europe is electrified, and this figure continues to increase, it is not advancing as rapidly as anticipated. In contrast, electrified tracks in the United States and Canada represent only around 1% of their respective networks. This can be justified by reports [3] that estimated costs of electrification are about twice as expensive in the USA compared with the European Union. The primary factors contributing to this situation are the high cost and technical challenges associated with full electrification. For example, electrifying infrastructure in tunnels or above road bridges poses significant technical and economic difficulties, making it financially impractical, even when considering only operational costs (excluding capital expenses), particularly for low-traffic railway lines. Moreover, according to European research objectives, freight transport \"still no shift from road to rail\" [4], but especially in Spain (Table 1). According to statistics [5], Spain lags behind most European countries in terms of freight transport by rail, performing well below the average. Additionally, the trends are negative, as can be observed in Table 1.  
Following the deregulation of the railway sector a decade ago, private operators now account for more than 45% of the market share (and over 50% in terms of ton-kilometers) [6]. This shift has led to a surplus of freight locomotives, particularly electric ones, within the fleet of Spain's traditional national railway operator, RENFE. Many of these surplus locomotives are either leased to private operators or decommissioned well before reaching their projected design lifespan (e.g., 4-5 Mkm). Overview of Energy Storage in Railway Power Systems (RPS)The technical feasibility of On-Board Energy Storage Devices (OESD) for trains is not a new concept and has been demonstrated by numerous passenger units and some shunting locomotives, which operated successfully for extended periods (e.g., in Germany from 1955 to 1995). However, these early implementations faced significant limitations, including restricted operating range, challenging track profiles, and the substantial weight of storage devices. While advances in storage technologies (such as reduced battery charging times and extended lifespans) have addressed some of these issues, a major challenge persists: the demanding operational conditions of railways, which often differ significantly from the standard applications of energy storage devices, as noted in the specialized literature [7].