National infrastructures are at risk from extreme weather events and random shocks. For example, the vulnerability of Britain’s national-scale rail infrastructure has been highlighted during extreme floods, and it seems likely that such risks will increase in future years. Given the importance of rail infrastructure, there is need for an improved understanding of the risks it faces. This paper provides a methodology to meet this need, by analysing the systemic risk to Britain’s rail infrastructure from a range of disruptive events. It first considers the range of events and processes which have the potential to disrupt operation of the rail network. A procedure is developed for assessing the relative criticality of different nodes and edges on the network based on the passenger traffic they carry. Two case study risk types (floods and traction system failures) are used to demonstrate how criticality assessment can identify parts of the rail network most at risk of causing substantial disruption to rail traffic, and therefore are most critical to maintaining national mobility. The paper concludes by considering implications of this analysis for investment decisions and the potential for transferring this methodology to other spatial or economic contexts.