Track21: Railway track research for the 21st century

Track21 is a major new research programme funded by the Engineering and Physical Sciences Research Council (EPSRC) with the aim of developing the fundamental science needed to bring about a step-change improvement in the performance of the UK’s railway track system.

Led by Professor William Powrie of the University of Southampton, Track21 brings together worldleading academics from the Universities of Southampton, Birmingham and Nottingham with key industry players to develop the scientific knowledge needed to inform not just incremental advances, but a step change in the way the existing track network is maintained and new lines designed and built.

The key research challenge addressed by Track21 is to develop improved understandings of the complex mechanisms of railway track behaviour governing stiffness, robustness, longevity, noise and vibration. It investigates railway track as a matrix of elements interacting with each other and with external factors, rather than as a linear series of individual components as has often been done in the past. The integrated picture this approach provides has the potential to transform the engineering, environmental and economic performance of track systems. The ultimate goal is to reduce deterioration rates and the requirement for maintenance substantially, while at the same time mitigating the environmental effects of noise, vibration and materials use. These are perhaps the most significant challenges facing railway systems today; if successful the research will lead to reduced whole-life costs and improved timetable reliability, together with the accompanying financial, environmental and customer service benefits.

 To meet the challenge, a coordinated programme of research is being carried out by a team of world-leading engineers and scientists from different disciplines and universities and key industry experts. The research team will apply their collective expertise to develop novel scientific solutions relevant to the experience and needs of practising railway engineers involved in design, maintenance and renewal of track systems.

Background: the research problem

Currently, the majority of the world’s railways – including all main lines in the UK – are on ballasted track. Although there have been developments in component specifications and materials, the principles of the system have changed little over the past 150 years. Ballasted track has generally been considered to offer the optimum solution in terms of construction cost, stiffness and drainage properties, and ease of modification: so although more highly engineered track forms have been used, ballasted track has been the technology of choice both for upgrades and for new highspeed lines including HS1. However, the limitations of ballasted track are becoming more apparent and more significant as the demands placed upon it increase. Recent trends towards more, faster and heavier trains have led to greater loadings, both in terms of maximum axle load and number of cycles. This tends to increase the rate of track geometry deterioration leading to higher than expected maintenance requirements and costs. Thus a transformation in track performance – by retro-fit measures for existing ballasted track, and possibly by an informed decision in favour of an alternative track system in the case of large-scale renewals – is essential if the Railway Technical Strategy (RTS) aspirations of reduced cost and increased capacity are to be realised.

The behaviour of ballasted track depends on the performance of a number of inter-related elements, and currently how these interact with each other is not sufficiently well understood to assess the impacts of new designs and specifications and external factors such as more onerous train loading and climate change. Track21’s research agenda is based on the premise that there are a number of such areas in which current knowledge is particularly inadequate and where further fundamental investigation is required.

Design rules for ballasted railway track that specify the optimum depth of ballast are traditionally based on custom and practice, and do not generally reflect the reality of complex loadings involving a rotation of the principal stress direction as trains pass. The response of the ground to train loading is not fully understood, and particular problems occur at transitions from normal ground onto hard substructures, and at complex track geometries such as switches and crossings – which between them account for less than 1% of route length in the UK, but about 20% of maintenance costs.

Despite recent advances, ballast is routinely treated as a continuum in track design when, in fact, it is a granular medium with large particles whose discrete nature has a major influence on the track mechanical response. External loads are carried through a network of interparticle contact forces. The stability and resilient stiffness of the ballast layer, and the rate at which permanent deformation accumulates, depend ultimately on the number of these interparticle contacts available to transfer forces. Although this, in turn, depends on the grading (i.e. the particle size distribution) of the ballast, at the moment there is no fundamental understanding of how ballast grading affects its resilient mechanical response, and ballast specification remains largely empirical. The development of such understanding is crucial for the rational and informed design of ballasted track that requires minimal maintenance even in conditions of heavy use.

The environmental credentials of current renewals practices are also questionable, since they result in the unnecessary replacement of millions of tonnes of ballast each year. Noise and ground-borne vibration too represent further potentially costly and politically significant environmental concern for railways. Track-radiated noise is usually greater than vehicle noise, and although modelling of noise generation is advanced, appreciation of the role of the track system and sub-base in vibration generation and noise propagation is at an early stage and a much fuller understanding is required.

By obtaining a better understanding in these problem areas the performance of railway track can be significantly improved. Enhanced understanding of the behaviour of the components, the interactions between them and the system response to external loading and environmental conditions will allow the development of more effective and efficient maintenance and renewal strategies, leading in turn to reduced costs, increased capacity and improved reliability. This would prepare the way for a radical overhaul of current railway track design (materials, specifications, buildability, maintainability) appropriate for both new build (e.g. HS2) and upgrades to meet current and future train loading requirements much more efficiently than is at present possible.

Research methods and approach

The behaviour of ballasted and more highly engineered railway track systems will be investigated by means of a series of inter-related experiments together with supporting mathematical and numerical modeling, field monitoring and observation. The research is managed in six Work Areas (WA’s) which, although highly interconnected, come online roughly sequentially as the work progresses through three stages, from developing fundamental understandings of the behaviour of the system elements, including their response to external factors; to evaluating how these can affect system performance; and finally understanding and predicting the engineering, environmental and economic benefits of integrating the new knowledge into practice.

The six WA’s are:

1. Railway foundations / sub-base The focus here is on understanding the behaviour of the sub-base, considering the responses of different types of soils and degrees of saturation to stress paths representative of train passage. The concurrent effects of future climate change on the behaviour of clay soils, especially embankment fills, in response to seasonal changes in pore pressure are also addressed.

2. Ballast and sleepers This strand considers new approaches to improving the performance of ballast and sleepers, and prolonging ballast life. The work includes understanding and manipulating the effects of ballast grading; techniques for improving ballast performance such as ballast bags, gluing and randomly oriented fibre or strip reinforcement; sleeper material and shape; and sleeper pads and ballast mats.

 3. Noise and vibration This looks at the effects of ballast and sleeper modifications and critical zone improvements identified in WA2 and WA4 on the environmental noise and vibration impacts of railways, together with potential mitigation measures such as ballast mats and sleeper pads. Effects on vibration generation and propagation of changes to the track system (WA5) and the distribution of subbase stiffness (WA1) will also be investigated.

4. Critical zone improvement Critical zones including switches, crossings and transitions onto hard substructures require considerably more maintenance than plain track. This stream of research will investigate and explain quantitatively the factors responsible for this behaviour, drawing on the outputs of WA1 to WA3, to develop guidance to improve critical zone performance and reduce maintenance requirements.

5. System integration Work Areas WA1 to WA4 will identify a variety of potential improvement methods applicable mainly to individual elements or specific aspects of performance. In WA5, the effectiveness of such improvements at a system level will be investigated and assessed through physical and numerical modelling, and hopefully field trials.

6. Performance, environmental and economic modelling This work area will develop models enabling the whole life economic and environmental costs and benefits of the sub-base, ballast and track system improvements identified in the previous WA’s to be quantified. Engineering performance and cost models will be developed to supplement existing approaches. Carbon and environmental impacts, and embodied vs operational vs whole life energy use, will all be considered.

Summary

The benefits of improving the performance of railway track systems are huge: maintenance costs would be reduced, network capacity increased and environmental impacts mitigated. All of these are essential to the development and operation of railway systems for the 21st century. To achieve these goals is a major challenge, which requires nothing less than a systematic quantitative understanding of the fundamental behavioural mechanisms of individual track components, and how they interact and respond to external factors to govern the performance of the track system as a whole. Despite recent developments, theoretical knowledge is incomplete and insufficiently integrated with what happens in reality. The Track21 team aims to remedy this, and in doing so to facilitate a transformational change in railway track systems research and practice – this will have substantial pay-offs for a wide range of stakeholders in the railway industry, for users of rail transport, for government and ultimately for the whole of society.

About the Authors

William Powrie FREng is Professor of Geotechnical Engineering and Dean of the Faculty of Engineering and the Environment at the University of Southampton. He is Principal Investigator for Track21. His main technical areas of interest are in geotechnical aspects of transport infrastructure, and sustainable waste and resource management.

Tony Leyland is Project Coordinator for Track21 and is Secretary of the Rail Research UK Association (RRUK-A). He is based at the University of Southampton.