For the past two years, Altran – in collaboration with Network Rail -have been developing COMPASS (DMWS), a system which provides an electronic degraded-mode working solution that includes trackside, train borne and central control and communication systems that communicate using wireless technologies. The COMPASS (DMWS) project team is carrying out security analysis against ISO/IEC 62443-3-3:2013 and Network Rail’s Security Assurance Framework to secure this system for use on the UK national rail network. This paper introduces the system and the security analysis process, and walks through the issues that influence the security architecture.