Samuel D. Bemment

Redundantly Engineered Track Switching for Enhanced Railway Nodal Capacity

Abstract Railway track switching provides necessary flexibility to a rail network. However, the inclusion of switches also introduces single points of failure and restricts system capacity through train control rules. Historically, switching practice has evolved with safety as the ultimate priority with minimal consideration given to how the design, and its associated control systems, detracts from maximum network capacity - primarily as switches have rarely been the limiting factor. Over recent years, capacity has become more critical. As rail systems are improved, the traditional design of track switch and control methodology represent an ever increasing portion of theoretical maximum network capacity which cannot be utilised. Herein, an example of this capacity limit is presented. The proposed HS2 (High-Speed 2) rail link in the United Kingdom, where metro-frequency services are proposed to run on 250mph lines, is pushing the boundaries of what current technology can offer. Some industries have resorted to functional redundancy to provide the fault tolerance necessary in similar mission-critical systems, without sacrificing safety. This paper builds upon earlier published work by evaluating novel track switching concepts incorporating functional redundancy in terms of the potential to boost capacity. Possible changes are discussed, and potential implications of those changes are quantified using a model of a typical HS2 node in Railsys. Maintainability and cost implications are outlined. Results demonstrate that a substantial revision of track switching practice can yield a large percentile increase in the capacity of the junction; in the HS2 case matching the capacity of the adjoining plain line.

Rethinking rail track switches for fault tolerance and enhanced performance

Railway track switches, commonly referred to as ‘turnouts’ or ‘points,’ are a necessary element of any rail network. However, they often prove to be performance-limiting elements of networks. A novel concept for rail track switching has been developed as part of a UK research project with substantial industrial input. The concept is currently at the demonstrator phase, with a scale (384 mm) gauge unit operational in a laboratory. Details of the novel arrangement and concept are provided herein. This concept is considered as an advance on the state of the art. This paper also presents the work that took place to develop the concept. Novel contributions include the establishment of a formal set of functional requirements for railway track switching solutions, and a demonstration that the current solutions do not fully meet these requirements. The novel design meets the set of functional requirements for track switching solutions, in addition to offering several features that the current designs are unable to offer, in particular to enable multi-channel actuation and rail locking, and provide a degree of fault tolerance. This paper describes the design and operation of this switching concept, from requirements capture and solution generation through to the construction of the laboratory demonstrator. The novel concept is contrasted with the design and operation of the ‘traditional’ switch design. Conclusions to the work show that the novel concept meets all the functional requirements whilst exceeding the capabilities of the existing designs in most non-functional requirement areas.

An Evaluation of Redundancy Concepts for Fault Tolerant Railway Track Switching

Abstract Railway track switching provides necessary flexibility to a rail network, but also introduces many single points of failure. Other industries have resorted to redundancy to provide the fault tolerance necessary to achieve specified safety and operational goals in similar critical systems, yet this approach has not yet been applied to track switching. This paper explores several system concepts for redundancy in track switching, and attempts a rudimentary cost/benefit analysis of each. Mechanical design is not considered, the paper being a feasibility study only. Finally, concept performance is evaluated in a case study at Derby Midland station (UK). The results show that providing redundancy which utilises only existing technology demonstrates some gains in operational availability. However, it is unlikely to be cost effective, and for redundancy in track switching to be a realistic prospect, novel designs and technologies may be necessary.

Improving the reliability and availability of railway track switching by analysing historical failure data and introducing functionally redundant subsystems

Track switches are safety critical assets that not only provide flexibility to rail networks but also present single points of failure. Switch failures within dense-traffic passenger rail systems cause a disproportionate level of delay. Subsystem redundancy is one of a number of approaches, which can be used to ensure an appropriate safety integrity and/or operational reliability level, successfully adopted by, for example, the aeronautical and nuclear industries. This paper models the adoption of a functional redundancy approach to the functional subsystems of traditional railway track switching arrangements in order to evaluate the potential increase in the reliability and availability of switches. The paper makes three main contributions. First, 2P-Weibull failure distributions for each functional subsystem of each common category of points operating equipment are established using a timeline and iterative maximum likelihood estimation approach, based on almost 40,000 sampled failure events over 74,800 years of continuous operation. Second, these results are used as baselines in a reliability block diagram approach to model engineering fault tolerance, through subsystem redundancy, into existing switching systems. Third, the reliability block diagrams are used with a Monte-Carlo simulation approach in order to model the availability of redundantly engineered track switches over expected asset lifetimes. Results show a significant improvement in the reliability and availability of switches; unscheduled downtime reduces by an order of magnitude across all powered switch types, whilst significant increases in the whole-system reliability are demonstrated. Hence, switch designs utilising a functional redundancy approach are well worth further investigation. However, it is also established that as equipment failures are engineered out, switch reliability/availability can be seen to plateau as the dominant contributor to unreliability becomes human error.