## 1 Introduction
To this end, the Internet of Things (IoT) paradigm [21] or, more precisely, the advanced system of machine-to-machine communication (M2M) [22], together with different front-end intelligent sensors [23], allows for the transparent integration of large numbers of heterogeneous external systems [24]. Thus, it can facilitate the development of new digital services and novel possibilities for their application, especially when transport safety and security are concerned [25]. Also, the development of communication systems according to 5G functionalities opens new possibilities in the field of ITS and communications between traffic entities and their environments, and such internet of vehicles (IoV) paradigm [22] is especially important in the field of autonomous and cooperative vehicle management [26]. Reference [27] provides an overview of the possible technologies, protocols, and architectures of intelligent systems based on the concept of vehicle communication systems, with an emphasis on a wide variety of communication types such as vehicle-to-vehicle (V2V), vehicle-to-infrastructure (V2I) and vice versa (I2V), vehicle-to-pedestrian (V2P) communication, and, generally speaking, any communication between a vehicle and a heterogenous communication node (V2X) [26]. Furthermore, future transport systems based on non-fossil vehicle propulsion technologies are also closely related to smart grid systems and 5G software-defined vehicle networks that have important roles in that field of research [28]. In particular, 5G communication networks offer distinct advantages in terms of connection speed and data transfer reliability (up to 20 times faster data rates for the same transmission quality) compared to the standard Global System for Mobile Communications-Railway(GSM-R) systems [29], and this low latency of 5G networks enables the implementation of various security functions over multiple heterogenous domains [30]. Thus, 5G communications have been suggested for the digitalisation of urban railways [31] and the telemetric monitoring and automation of railroad networks [32], along with sophisticated applications in the accurate positioning of railway traffic entities in real time [33]. The latter is significant in the context of railway traffic safety, especially when atmospheric and weather conditions at micro-locations are taken into account [34]. This primarily concerns railway traffic incidents related to weather conditions [35], but it may also refer to the feasibility of the scheduled transportation by rail under adverse weather conditions [36]. The application of smart remote sensor networks facilitating timely and structured information about atmospheric conditions may offer an additional advantage in the planning of railway transportation, as indicated in [36], which is of particular importance when considering increased weather condition volatility due to climate change [37]. Having the above issues in mind, this paper presents a review of state-of-the art and emerging technologies needed for the future development of smarter, safer, greener, and more sustainable railway transportation, while also outlining its inherent challenges. In that respect, Section 2 deals with energy efficiency and advanced propulsion technologies, such as the hybridisation of locomotive diesel-electric powertrains, and other measures of on-board locomotive electrification, including purely battery-based propulsion and the utilisation of hydrogen fuel cells. Section 2 also discusses the hydrogen economy and logistics in the context of railway transportation, as well as the utilisation of alternative fuels such as biofuels, and the production of synthetic fuels for transport using renewable energy sources. Section 3 deals with advanced information and telecommunication (ITC) systems based on long-term evolution (LTE) 4G/5G radio networks (RNs) suitable for railway applications and compares them with the current GSM-R communication standard.