The Role of Data in Safety-Related Railway Control Systems

In the production of a computer-based safetyrelated system, it is common to partition the hardware and software elements into a system architecture. The software part of such an arrangement will generally include both the instructions that are executed by the processor(s), and the data that is used and produced by these instructions. In some cases, a large amount of static or configuration data forms an essential element within the system and plays a vital role in ensuring its correct operation. While data is subject to errors, experience shows that it is often not subjected to safety analysis techniques such as hazard and risk analysis. Data is often treated in a totally unstructured manner (often making error detection difficult) and very rarely is fault tolerance used to protect the system from data errors. To illustrate the role and importance of data in safety-related systems, this paper looks at the data associated with a railway command and control system. Such a system has a range of safety-related functions, and must also operate in the context of other safety, protection and business planning systems. The paper considers typical data errors associated with the railway environment and proposes the early definition of a system data architecture, which will allow the application of safety analysis techniques such as HAZOP.