Interoperability of the rail system within Europe is a key to its competitiveness. It aims at creating a rail network allowing a transport that is safe, compliant with the required performance level of the lines, and which does not necessitate train transfers. This requires the compliance with a set of rules, of technical and operational conditions which ensure that all the safety requirements are met. The main proposition is to contribute to the implementation of a European system for railway signalling called ”European Rail Traffic Management System” (ERTMS) using a methodology based on B framework tools. The idea is to assist the writer of the national operating rules. The national operating rules are expected to define the precise implementation of the high level ERTMS Specification Requirement System (SRS). They fulfils the national rule and refines the ERTMS specification in the context of a particular infrastructure. Some parts of the specification may be considered as irrelevant in the context of some particular infrastructures. A second step consists in refining the needed behaviour so that the national safety conditions and the ERTMS can be proved to be fulfilled. The methodology proposes to represent all the needed information using the B formalism in a three stepped methodology: The first step models the useful part of ERTMS specification in the context of the considered infrastructure. A requirements model is obtained. The second step consists in enriching the existing model with a precise description of the functioning of the infrastructure. More precisely, the interlocking implementation and the Automatic Train Protection (ATP) is taken into account and a process model is obtained. The third step consists in consistency checking between the two first models. The second model should be a refinement of the first one, because it is a particular instantiation of the ERTMS requirements. When it is not the case, some proof obligations will not be validated by the proof assistant included in the B framework. Then, the writer of the operating rules can try to introduce new logical propositions so that the proof obligations are fulfilled. When these three tasks are achieved, the writer of the rule has the logical proof that his operation rule is a national instance of the ERTMS SRS. The focus of the paper will be mainly on the second step.