# Communication Transport Protocol Strategies for Rail Applications
## 1 Introduction  
The rail sector is expected to increase enormously in upcoming years. The forthcoming Digital Transformation will be based on reducing any kind of carbon footprint, and the transport sector will be one of the most impacted sectors. In fact, rail transport will be the favored method to move people and freight worldwide due to its reduced carbon dioxide (CO\({}_{2}\)) emissions. The increase in the adoption of rail in transport has raised interest worldwide, and this will bring to some challenges for rail operators. They will be expected to operate more efficiently, thus increasing the number of trains per kilometer, as well as to guarantee safety and service quality both for passengers and freights. A few years ago, safety and optimization operations for rail traffic management and to avoid train collisions were performed by the Automatic Train Protection (ATP) system. This is a complex architecture based on track-side equipment placed at about 1-2 km in the rail path(and usually at 1.2 km in critical points such as rail exchanges and station approaching), which signal to the train driver how to adapt the train speed [1]. This system is not very reactive to signals, which is a further problem, and it is standard in all rails crossing national borders. Recent papers have investigated transport safety, including for example, the high in-train compressive forces that could determine train derailment [2]. Nevertheless, a first step in innovation in this field is the development of a Single European Railway Area in order to achieve fully interoperable rail in Europe. The first step has been the European Rail Traffic Management System (ERMTS) set up by the European Union in the Directive 96/48/EC [3] to specify procedures and technologies, and Directive 2001/16/EC [4] for the interoperability of the trans-European conventional railway system. This is based on the European Train Control System (ETCS) and the Global System for Mobile communications for Railway (GSM-R) and has been deployed for mainlines. GSM-R has the same characteristics of the classical GSM but it transmits on a dedicated frequency spectrum assigned to the rail operators. Moreover, dedicated base stations have been deployed by rail operators to provide connectivity services along their lines, thus supporting rail applications. Supported services are: (i.) voice communications (for rail staff and emergency calls), both for point-to-point and group calls, and (ii.) low-data rate services for railway operations (ETCS L2/L3 operations). Unfortunately, this strategy is no longer bearable due to the management of the technological infrastructure and costs. Moreover, GSM will be guaranteed until 2030. Two main proposals have been raised. One is proposed and supported by the International Union of Railway (UIC) and by the European Union Agency for Railway (ERA) by 2013. In fact, in 2015, the Technical Committee Rail Telecommunication (within European Telecommunications Standards Institute, ETSI) and 3 rd Generation Partnership Project (3 GPP) started to work on the Future Railway Mobile Communication System (FRMCS). This study concerns (i.) the choice of technology or technologies for communications; (ii.) the assigned spectrum for future radio communication system for railway, and (iii.) the required additional investment on new radio sites with respect to GSM-R radio sites. The second initiative has been supported by the European Shift2 Rail Joint Undertaking (S2R-IU), in which the concept of Adaptable Communication System (ACS) is proposed. While FRMCS is managed by telecommunication operators, ACS has the Over-The-Top (OTT) approach, which implements the bearer independence and supports connections over multiple networks. Other works proposed the adoption of Multi-Path Transport Control Protocol (MPTCP) to solve this problem. In [5], some experimental results have been provided, but only for increasing the overall data transfer throughput by utilizing the multihoming capabilities of communicating nodes.