A railway interlocking system is one example of a critical system, and, therefore, it must have a high level of reliability in order to avoid problems that may result on the loss of people's lives. However, many railway systems are still specified using historical relay-based diagrams, whose analysis are made by human inspection, which is error prone. Relay-based diagrams are specified by nodes and cables in a graphical manner, which resemble undirected graphs. This paper presents a framework for the specification of relay diagrams in a formal language, B-method, based on the specification of a graph and its properties. The use of a formal language allows one to prove the correctness of these railway interlocking systems regarding structural properties. This framework has been evaluated by the specification of a case study.