# Towards the Internet of Smart Trains:
## 1 Introduction
However, factors such as operational inefficiency, the lack of infrastructure and interoperability, high initial cost of deployment, and the integration complexities over legacy systems and the network, may hinder the rail industry growth. Moreover, legacy infrastructure, aging communications systems, and the slow adoption of automation and protective technology in this scenario pose enormous safety risks. Related to the issues of safety and connectivity is security. As rail systems rely more and more on wireless connectivity, they become more vulnerable to outside interference, intrusion and cyber attacks. The consequences of even a small disruption become particularly severe as trains become more powerful, carry more passengers, and travel faster. Systems that are mission-critical for safe operation can be compromised by a simple electronic device or a small piece of malicious code. When passenger safety and lives are at stake, strong security becomes a fundamental requirement. Nowadays, the main challenges when enhancing rail transport can be summarized as [5]:  
* Increase efficiency and competitiveness: railways face ferocious competition from other modes (for example, the road sector provides attractive, cost-effective, reliable, flexible, and convenient door-to-door transport of freight and passengers across borders). In Europe, the challenge is further increased by a fragmented rail market, with numerous national systems for rail signaling and speed control. Thus, interoperability represents a key challenge for the free flow of rail traffic. * Reduce rail noise and vibration, particularly in urban areas. * Reduce greenhouse gas emissions. Although rail transport compares favorably to other transport means in terms of environmental impact, it can be further improved. * Safety and security [6]: rail safety in the European Union (EU) is among the highest in the world. Rail incidents (accidents, terrorism...) are not frequent and cause a relatively low toll of deaths, but often involve a substantial number of people. In order to maintain and enhance security, interoperable and harmonized safety standards are required. * Reduce operation and maintenance costs, augment the capacity of the rail network. As it will be explained in the next section, considering the diversity of scenarios, the network architecture should include different types of access networks and technologies at different frequency bands in order to fulfill different operational requirements. This review introduces a comprehensive analysis of the evolution of the communications in the European railways since the deployment of GSM-R. It examines the different alternatives proposed over time and how railway requirements, specifications and recommendations have evolved in recent years. Unlike recent literature, the main contribution of this work focuses on presenting a holistic approach to IIoT applied to railways with a thorough study of the most relevant technologies (like the communications network). Its aim is to envision the potential contribution of enabling technologies for revolutionizing the industry and confront today challenges. The rest of this article is organized as follows. Section 2 provides a brief introduction of the main railway scenarios, examining the communications technologies and architectures used nowadays. Section 3 reviews the basic railway-specific requirements and services offered by GSM-R. Section 4 analyzes the factors that influence the deployment of LTE, and what is necessary to comply with the specific requirements of railway services. The advantages of the newest generation of communications systems for the railway environment are also explained together with the roadmap to ensure a satisfactory migration from GSM-R to LTE-R. Section 5 describes the rise of industrial IoT and the paradigm of Internet of Trains. Furthermore, the main industrial developments are described. Section 6 reviews the main short and medium-term IIoT-enabled services for smart railways. Finally, Section 7 is devoted to the conclusions.