Managing Complexity in Safety-critical Railway Signaling Systems using Simplex Architectures

With the increasing reach and applicability of software systems in railway infrastructure, their complexity and the demand for constant change increases. Main factors include the use of unreliable COTS components, technical obsolescence, increasing centralization, international harmonization of technical and operational standards as well as IT-security concerns. Statically assessing the functional safety of a software-based railway system (offline assurance) is thus insufficient to maintain continued operation.Applying simplex architectures on multiple layers of the system architecture can alleviate this problem: Using a simple decision logic component, simplex controls the complexity of an evolving and multi-faceted system while maintaining the level of dependability that is required for safe passenger transport. We argue that the introduction of online assurance marks a paradigm shift in the railway domain, where the system safety today is guaranteed by expert assessments.