This paper has three main objectives. The first objective is to summarize the requirements for RAM (Reliability, Availability, and Maintainability) parameters of European Rail Traffic Management System (ERTMS) defined in the railway standards. The second objective is to emphasize that the RAM requirements should be considered at the ERTMS SoS level. The third objective is to highlight major issues, when dealing with ERTMS SoS, which are not treated or clearly defined in the railway standards. Indeed, the RAM parameters definitions do not take into account all types of uncertainty in failure data and human failures, and do not propose specific methods to obtain failure data from experts' opinion. In this work, a number of methods have been proposed to deal with these issues.