Architecting railway systems for resilience

Author: Luke Church
Day: Aspect Day Two
Session: Architecture

The primary purpose of a railway is to safely and reliably transport people and goods; resilience to disruption is crucial to railways achieving this purpose. A railway is a highly complex system, consisting of multiple interconnected sub-systems; enhanced resilience can be achieved by analysing the architectures of railways systems, for example signalling systems, which are the subject of analysis in this paper.Resilience is defined as the capacity to recover quickly from difficulties. In the context of a railway this includes maintaining a normal service during minor disruptions and ensuring graceful degradation and swift recovery from major disruptions.There are four basic principles of resilience: capacity, flexibility, tolerance and cohesion. Railway systems are highly effective at achieving resilience through capacity, or more specifically redundancy. Examples include physical redundancy through duplicating components to avoid single points of failure, as well as functional redundancy such as secondary train detection in metro systems. Historically railway systems have had less focus on achieving resilience through flexibility, tolerance and cohesion. This paper explores how systems outside of the railway achieve resilience through these other principles, and propose means of applying these principles to railway systems to improve resilience.Insights can be gained into existing railway systems by reverse architecting the system to understand its context and interactions between component parts. Conversely, forward architecting can be used to develop system architectures of new railway systems to ensure they best meet the needs of the user. This paper will present case studies of both approaches; reverse architecting an existing railway system to understand how it achieves resilience and insights into how resilience may be improved, as well as how a new railway system is architected for resilience by design.Threats to the railway from sources such as cyber-attacks and extreme weather are increasing. At the same time more train services are being introduced through modernisation of infrastructure, which reduces the margin to withstand disruption through absorption, and limits are being reached in what can economically be achieved through redundancy. Railway performance can be improved by architecting new and existing railway systems for resilience, taking into consideration the lesser utilised principles of flexibility, tolerance and cohesion within railway systems.