## 2 Measures for Improving Railway Transport Energy Efficiency and Sustainability
### Utilisation of Battery-Based Locomotive Propulsion  
Although the research on battery-electric locomotives and their commercial applications is definitely on the rise, this topic has been researched rather modestly to date [55]. Advanced batteries, such as those based on sodium-metal halide and lithium-ion chemistries, were suggested in [56] as possible alternatives to diesel engine-based propulsion, especially when light duty hauling (power ratings less than 400 kW) is considered, with the possibility of periodic kinetic energy recuperation cycles, such as in mining operations [56]. This claim is further corroborated by the authors of [57], who propose the utilisation of a high discharge power and high-energy-density lithium polymer batteries for a battery-electric shunting locomotive. In fact, viable prototypes of purely battery-electric locomotives were recently introduced for rail yard switching operations and light to medium cargo hauling in industrial railway sidings and logistics terminals [58]. The authors of [59] showed that retrofitting the locomotive propulsion to a fully battery-based energy source may even be accomplished by using new developments of \"mature\" battery technologies, such as lead-acid batteries with activated carbon-based negative electrodes. These so-called ultra-batteries [60] supersede conventional lead-acid batteries in terms of pulsed power capabilities and service life while retaining their favourable temperature stability and ease of maintenance, and they have already been proposed for stationary energy storage use [61]. A lithium-titanate battery system was recently used to demonstrate the viability of battery-based propulsion for passenger multiple units in [62]. On the other hand, the authors of [63] investigated the possible use of battery-based propulsion for the so-called \"last mile\" regime for a freight locomotive as an alternative to the so-called \"electro-diesel\" configuration (with a diesel generator as a secondary energy source), which is suitable for rail transportation over electrified and non-electrified railway routes. The authors of [63] also considered the possibility of using battery recharging waystations, whose benefits and optimal placement along the railway route were recently investigated in detail in [64]. A techno-economic study of freight railway electrification by means of an overhead line, hydrogen fuel cells, and batteries for the railway lines in Norway and the USA [65] has shown that hydrogen fuel cells and batteries can be competitive technologies provided that renewable energy sources are available for charging and hydrogen production. Moreover, a comparative analysis of a 3 kV direct current (DC) electrification system and trains with on-board battery-ultracapacitor energy storage was carried out as a part of the preliminary techno-economic study in [66]. The study showed that more favourable economic indices would be obtained with energy storage-equipped passenger trains rather than investing in the electrification of lengthy railway routes with infrequent traffic. Owing to their exceptionally long cycle life of over \(10^{6}\) charge-discharge cycles characterised by deep discharges and large discharge rates [67], ultracapacitors have shown great potential in terms of supplementing batteries and hydrogen fuel cells for high-power/short-duration (pulsed) loading regimes [68]. This has been illustrated for both stationary energy storage systems in microgrids [69] and those with potential use in transportation applications [70], wherein LiFePO\({}_{4}\) batteries are considered as good candidates in terms of energy density and operational safety [48]. Moreover, it seems that the energy densities of the next generation of commercial ultracapacitors are approaching the energy densities of electrochemical batteries, thus allowing for prolonged loading compared to ultracapacitors from just a decade ago [67].