## 2 Measures for Improving Railway Transport Energy Efficiency and Sustainability
### Electric Powertrain Featuring Hydrogen and Other Types of Fuel Cells  
As mentioned above, the techno-economic study in [65] showed that hydrogen fuel cells and batteries can be competitive technologies in rail freight hauling when used in synergetic relation with renewable energy sources, while at the same time achieving zero net emissions of greenhouse gases. On a smaller scale, this kind of fuel cell plus battery hybrid power source was investigated for the case of a hypothetical 200 kW hydrogen fuel cell stack coupled with a nickel-metal hydride (NiMH) battery for tramway propulsion in [78], which also provided a component sizing and parameterisation methodology based on a realistic driving cycle on a metropolitan tramway line. For larger-scale, high-power applications, i.e., those suitable for freight haul and passenger transport over greater distances, reference [79] demonstrated that solid oxide methane-fed fuel cells in combination with a gas turbine, steam, and ammonia-organic Rankine cycles can be used to produce power in excess of 2900 kW, with waste heat recovery used for additional power production and the heating of the passenger compartments. Moreover, the key advantage of solid oxide fuel cells is that, apart from methane, they can utilise other gaseous fuels for electricity generation such as ammonia and hydrogen, which is beneficial from the standpoint of their practical use [79]. Polymer electrolyte membrane fuel cells were proposed in combination with a suitable energy storage system to power an existing railcar as an eco-friendly solution for short- to medium-length non-electrified railway lines characterised by an altitude difference between the embarkation and arrival points [80]. The possibility of using fuel cells for sustained high-power output has motivated the design of a prototype fuel cell shunting locomotive to avoid diesel engine-related emissions of particulates and nitrogen oxides and acoustic noise emissions [81]. On the other hand, the authors of [82] investigated the possibility of retrofitting the conventional passenger train with hydrogen fuel cells as a power source to be used over a non-electrified  
Figure 2: Diesel-electric and battery-electric 1.6 MW locomotive combinations investigated in reference [76] and main results in terms of journey time, fuel consumption, and electric energy expenditure for a mountainous railway route with respect to freight train load. railway route, with the main advantages being reduced energy consumption and the elimination of greenhouse gas emissions, while at the same time modernising the current rolling stock and increasing the overall passenger comfort. A similar study carried out in [83] indicated that fuel cells may provide a viable low-emission solution for a demanding railway route characterised by steep slopes if they are used with a suitably sized electrical energy storage system. A comparison of three characteristic fuel cell-powered railway vehicles presented in [84] indicated that such solutions are indeed technically feasible, with the main concerns being their relatively low payloads and the availability of hydrogen fuelling infrastructure. Even though hydrogen fuel cell trains are still emerging technologies, which are yet to appear in commercial transportation roles, the costs of hydrogen generation are showing clear trends towards becoming cost-competitive to conventional fuels [85]. For example, the results of a study presented in [86] indicated that switching from diesel-based train propulsion to that based on hydrogen fuel cells would result in an annual saving of 9.5 million litres of diesel fuel, as it would be substituted with 2198 tonnes of hydrogen annually. This study also indicated that the cost of hydrogen production by means of electrolysis from renewable energy sources would amount to approximately EUR 6.40/kg of hydrogen, which makes this a competitive alternative to conventional fuels.