## 2 Measures for Improving Railway Transport Energy Efficiency and Sustainability
### Alternative Fuels Utilisation  
As indicated in [87], alternative fuels may play vital roles in the future 100% renewable transportation systems. Adopting the tenets of a circular economy and the utilisation of alternative fuels for heavy machinery in railways was identified in [92] as one of the potential means to mitigate environmental impacts. Biofuels, such as bioethanol, may be used alone or within gasoline blends, thus contributing to the reduced utilisation of petroleum-based fossil fuels [93]. Waste oil from the food industry has already been recognised as a source of raw material for the manufacturing of diesel fuel substitutes [94] that can easily be adopted for use in diesel-based railway traction [95]. Naturally, bioethanol and bio-diesel production would result in an increased use of arable land, especially when considering the production of corn (maize) and soy as precursors for ethanol and bio-diesel production [96]. However, the life cycle assessment (LCA) study carried out in [97] indicated that biofuels in general may have the lowest cumulative environmental effect compared to other conventional propulsion technologies. Another promising technology for cleaner transportation is that based on bio-methane and biogas, which can be used to power both light and heavy-duty transport [98]. Reference [99] has already shown that such an approach utilising gas engines instead of diesel-based ones in combination with battery energy storage may lead to significant fuel savings and a consequent reduction in carbon emissions. In that respect, the utilisation of dual-fuel locomotives may benefit from the existing distribution infrastructure for liquefied natural gas (LNG) [100], with one of the key benefits being that the utilisation of gas en gines results in significantly fewer particulate matter emissions [101]. However, one of the disadvantages of LNG (and other liquefied gases) use is the need for large-scale cryogenic gas storage [102]. On the other hand, the authors of [103] assert that massive transportation electrification is essential to utilise renewable energy sources to their maximum potentials. According to [103], the inevitable excess power production from renewables that cannot be immediately absorbed by the energy and transportation sector should be utilised for hydrogen and synthetic fuel production (the power-to-x concept mentioned earlier). With dimethyl ether (DME) having been recognised in [103] as a likely substitute for diesel fuel, its production and subsequent utilisation within conventional railway traction based on internal combustion engine technologies may lead to a significant reduction in railway systems' carbon footprint. The power-to-gas concept and the logistical aspects of synthetic gas distribution pathways were extensively investigated in [104], whereas reference [105] presented a novel process chain for the sustainable production of synthetic fuels based on renewables, as illustrated in Figure 3. It is based on the use of renewables for the generation of electric power, which is subsequently used for water electrolysis and hydrogen production. This hydrogen can then be stored within hydrogen fuel cells or used for synthetic hydrocarbon production by combining it with carbon dioxide reclaimed from the atmosphere using catalytic processes. Reference [105] also points out that railways may likely be able to achieve the necessary economies of scale, especially when hydrogen infrastructure is considered, while according to [106], the cost and complexity of alternative fuel infrastructure for railways could be much lower than that for road vehicles, particularly due to a priori known schedules (timetables) and traffic volumes. A detailed comparative analysis of different alternative fuels and their production paths was also carried out in [103], and their advantages and disadvantages in terms of economic and infrastructural barriers are presented in Tables 1 and 2.