We present a process for the development of safety and security critical components in transportation systems targeting a high-level certification (CENELEC 50126/50128, DO 178, CC ISO/IEC 15408). The process adheres to the objectives of an " agile development " in terms of evolutionary flexibility and continuous improvement. Yet, it enforces the overall coherence of the development artifacts (ranging from proofs over tests to code) by a particular environment (CVCE). In particular, the validation process is built around a formal development based on the interactive theorem proving system Isabelle/HOL, by linking the business logic of the application to the operating system model, down to code and concrete hardware models thanks to a series of refinement proofs. We apply both the process and its support in CVCE to a case-study that comprises a model of an odometric service in a railway-system with its corresponding implementation integrated in seL4 (a secure kernel for which a comprehensive Isabelle development exists). Novel techniques implemented in Isabelle enforce the coherence of semi-formal and formal definitions within to specific certification processes in order to improve their cost-effectiveness. 1 Motivation Use of formal methods as validation technique for certification of safety and security critical systems is sometimes regarded as counterproductive to industrial development processes, even for having an advantage over competitors. This holds for the railway-industry (following CENELEC 50126/50128), the avionics (DO 178 B/C) or the industry of security critical components (Common Criteria ISO 15408). A major reason for this reluctancy is the perception that these techniques are too complex to apply, require high-skilled contributors and therefore is time-consumming and not well mastered. This contributes to the fact that regulators speak of a " certification crisis " [10] which, in the case of CC 15408, is reflected by only a handful EAL7 (level requiring formal methods) certifications after 25 years of the standards existence... This research work has been carried out in the framework of IRT SystemX, Paris-Saclay, France, and therefore granted with public funds within the scope of the Program " Investissements d'Avenir " .