This chapter presents an approach for certified design of railway critical systems. This approach, which realizes the software development cycle, relies on metamodeling architecture and model-transformations. It combines semi-formal UML models and formal models in order to check, proof and generate code by refinement; we use the process algebra FSP to check the dynamic behavior and B to generate proved code. Initially, the authors select an UML subset, which could be uses to model the key aspects of critical systems. Then, from this subset, the authors perform projections to obtain B and FSP models which are exploited by tools for checking, refinement and proof.