Resumen: The electromagnetic interferences (EMI) are threats that affect the reliability of the railway signalling systems. Consequently, the identification of the reliability requirements dependent on environment conditions is a major issue for signalling systems designers, and therefore for evaluators, and testing and certification bodies. Signalling systems work in the complex and heterogeneous railway environment, where low power electronics have to work together with high voltages and currents from trains and railway infrastructure. This chapter presents the relationship between the railway electromagnetic interoperability and the reliability assessment by analyzing the signalling systems and the associated inter-dependencies with other components of the rolling stock. It is composed of two main sections; the first gathers an exhaustive state of the art approach to the issue of electromagnetic interoperability and railway industry. This subsection steers towards the combination of electromagnetic interferences and the signalling systems present in the rolling stock noise environment. That is the basis of the second section that finally sets how to establish the reliability requirement for a communication path in this environment. This requirement is established because of the electromagnetic noise environment, as well as the radiated and conducted fields, which are a combination of all the surrounding threats a focused railway system has to face.