## 2 Measures for Improving Railway Transport Energy Efficiency and Sustainability
### Hybridisation of Conventional Diesel Engine-Based Locomotive Powertrain  
The needs of industry and society for cheap energy continue to be largely covered by fossil fuel sources, and this is becoming an increasingly complex task in the face of highly fluctuating prices for oil and its derivatives. These arguments are key motivations for the massive electrification of the transport sector, which is mostly pronounced in road transport through the introduction of hybrid electric and fully electric vehicles [4]. Such advanced propulsion technologies, having enabled the penetration of electrified vehicles in the transport sector, have also been recognised as key factors for the development of intelligent transport systems [38] and their integration within the smart grid [39], especially when considering the increased participation of renewable sources in the overall energy balance of modern society [40]The fully electrified railway traction system is based on the distribution of DC or AC electrical power through the overhead conductor (or third rail in some cases), thus supplying low voltage (up to 800 V) or medium voltage (up to 25 kV) to the fully electric locomotive [41], whose main advantage over their diesel-electric counterparts is in a higher power-to-mass (power-to-weight) ratio [40]. According to reference [42], a notable portion of existing railway tracks characterised by lower traffic density and rather lengthy routes have not yet been electrified due to the rather high investment costs of electrification [43]. Therefore, a significant number of diesel locomotives is still retained in the fleets of national and private railway companies [44]. However, the use of diesel-electric locomotives requires significant allocations for diesel fuel. Therefore, there is a trend in the R&D of hybridised diesel-electric propulsion [45] with a special emphasis on the introduction of energy storage systems based on advanced large-capacity electrochemical batteries. In that respect, special attention should be given to the safety aspects of battery application, where high-temperature sodium-nickel chloride (ZEBRA) batteries [46] and lithium batteries such as those using lithium iron phosphate (LiFePO\({}_{4}\)) and lithium-titanate (LTO) chemistry stand out from the safety point of view [47]. According to the simulation study in reference [48], such advanced batteries were successfully applied in the hybridisation of a diesel powerplant in an isolated production plant micro-grid, indicating significant potentials for diesel fuel savings and a reduction in greenhouse gas emissions, amounting to over 12% compared to the non-hybridised microgrid case. Consequently, it is realistic to expect that the hybridisation of diesel locomotive propulsion (i.e., by installing a battery storage system of sufficient capacity and power ratings) could reach similar levels of fuel efficiency. In that respect, reference [49] reports up to 20% of fuel savings for certain operating modes, with similar reductions in greenhouse gas emissions and other pollutants [50], and a real possibility of reducing the acoustic noise emissions of the locomotive due to the use of stored electricity within the battery with a lower power output from the diesel engine [51]. Optimising energy consumption in rail transport is a multidimensional nonlinear problem subject to technological traffic constraints, especially when an increased degree of autonomy is considered, such as in the case of autonomous vehicles. Namely, their operation is characterised by additional safety constraints in terms of near objects' proximity warnings and automatic collision avoidance, which mandate driving mission altering in real time. Current trends in railway energy saving research have focused on driving strategy, propulsion, and energy storage systems [52], whereas reference [53] provides a preliminary solution for predictive monitoring and control system adaptation to optimise the energy consumption of an autonomous railway vehicle.