Specifying Software Requirements for Safety-Critical Railway Systems: An Experience Report

[Context and motivation] Software safety requirements are fundamental in the definition of risk reduction measures for safety critical systems, since they are developed to satisfy the system safety constraints as identified by mandated safety analyses. It is therefore imperative that the requirements are defined clearly and precisely. [Question/Problem] We describe our experiences in introducing a safety compliant method of writing safety software requirements for railway projects in a distributed organization. Our goal was twofold, to develop requirements specifications that comply with the EN 50128 standard and that are understandable by the persons involved in the software development. [Principal ideas/results] We introduced methods to transform natural language requirements to functional requirements described as scenarios, sequence, use-case and state-machine diagrams. [Contribution] Our experience shows that new ways of expressing requirements, even if proper to solve technical issues such as compliance with standards, bring other challenges to the organization like people’s reluctance to changes in working routines and process updates.