## 1 Introduction  
The European Union's long-term goals are based on green energy transition. Thus, the European Commission has adopted a package of measures aimed at energy efficiency improvements, along with legally binding guidelines for meeting the objectives of the Paris Climate Agreement [1]. According to those guidelines, at least 32% of energy consumption in the European Union (EU) by 2030 must be from renewable sources. Member states, including the Republic of Croatia, need to ensure that at least 14% of fuel (energy) in the transport sector is secured from renewable sources, and the overall energy efficiency would need to be increased by 32.5% by 2030 [2]. Obviously, the strategy for the decarbonisation of the energy sector also causes significant changes in the transportation sector in terms of usable energy sources, which, in turn, is reflected upon the propulsion technologies currently in use, particularly in railway transport [3]. Specifically, 20% of the global consumption of fossil fuels is currently attributed to the transportation sector [4], which, in turn, makes it the second largest carbon dioxide (CO\({}_{2}\)) emission source [5] contributing significantly to the alarming increase in atmospheric CO\({}_{2}\) concentrations [6] and associated greenhouse effects [7]. The contribution of railway transportation to the overall emissions of greenhouse gases (GHGs) varies depending on the country, with 4% of the overall GHG emissions share having been reported in [8]. The EU Strategic Program for Transport Research and Innovation developed the first long-term strategic approach to prepare for the envisaged transportation system transition in terms of research and innovation that combines innovative low-carbon technologies with connected and automated transport services and smart mobility [9], wherein a multitude of options is currently available for railway sector decarbonisation [10]. The need for a transition to more autonomous and connected transport has been identified as a necessary condition for achieving higher levels of efficiency and the decarbonisation of the transportation sector. According to the European Commission (EC), the goal is that by 2030, high-speed rail traffic will double across Europe, and this is planned both for urban and intercity collective travel [9; 11]. For journeys of less than 500 km, carbon-neutral automated mobility for smaller groups of people or goods should be available [11]. Also, the EC document [12], \"...Digitalisation and robotisation in the field of the mobility of people and the transport of goods provide society with several potential benefits such as better accessibility and convenience for passengers, efficiency and productivity for logistics, improved traffic safety and reduced emissions. At the same time, there are concerns relating to safety, security, privacy, labour and the environment...\", recommend that intensive research and development (R&D) of suitable solutions for intelligent transportation systems (ITSs) needs to be carried out. This need for additional R&D is especially emphasised in the fields of communication systems and sensor networks to minimise the risks of deploying these new technologies [13]. In particular, demands for the accelerated digitalisation of the transportation sector result in extremely large amounts of data, which mandate the utilisation of cloud computing. On the other hand, the implementation of long-term evolution (LTE) or fourth and fifth generation (4G/5G) mobile communication networks [14] and different remote (networked) sensor nodes [15], as well as robotics [16] and artificial intelligence (AI) in logistics [17], are seen as key factors for further advances in digitalised and automated railway transportation [18], including autonomous rail vehicles [19]. Steady progress in the development of sensor networks is visible in all domains, especially in the field of transport. Therefore, the effectiveness of communication equipment is particularly important due to requirements on transport volume and speed [20].