This article describes an approach that, starting with a overview of the system, leads to a detailed interface specification in clearly defined steps. The approach has been developed over the course of many technical discussions in collaboration with railway engineering experts of Deutsche Bahn AG. It is especially suitable for creating high-quality specifications in the complex environment of railway engineering. The procedure has become standard in DB Netz AG’s NeuPro project. Due to its clear structure, the approach is claimed to be suitable for major projects with large teams.