Think outside the box: new materials, design application, and performance of railway turnout bearers

Railway turnout systems are a main feature in railway tracks. There are two types of turnout support structures, which can be designed to be either a slab or a cluster of discrete bearers. The choice of turnout support system depends on asset management strategy of the rail operators or maintainers. This paper will focus on the discrete turnout bearer system where turnout alignments, settlement and deformation are comparatively more flexible and retrievable by routine ballast tamping. It embraces the consideration of new materials for use as a turnout bearer. Traditional bearer materials are timber, steel, and concrete. Recent developments have resulted in adoption of new materials, such as ‘fibre-reinforced foamed urethane (FFU)’, ‘geopolymer concrete’, ‘recycled polymer’, and ‘CarbonLoc composite’. The aim of this paper is to present the criteria, fundamental and multi-disciplinary issues of bearer design in a grillage turnout system. Some of new materials’ characteristics are illustrated to provide an in-depth understanding into the static and dynamic performances of a turnout over its life cycle, as well as the benefits of new materials in reducing the depth of bearers while maintaining stability. As a case study, a full-scale trial to investigate in-situ behaviours of a turnout grillage system using an alternative material, ‘fibre-reinforced foamed urethane (FFU)’ bearers, is presented. The field trial has been carried out at a complex turnout junction under heavy mixed traffics at Hornsby, New South Wales, Australia. Influences of the FFU bearers on track geometry (recorded by track inspection vehicle ‘AK Car’ and based on survey data), track settlement, track dynamics and acoustic characteristics are highlighted in this paper. Operational train pass-by measurements were analysed to evaluate the effectiveness of the replacement methodology. Comparative studies show that the use of FFU bearers generates higher rail and sleeper accelerations but the damping capacity of the FFU help suppress vibration transferring onto other track components. The survey data analysis suggests a small vertical settlement and negligible lateral movement of the turnout system. The static and dynamic behaviours of FFU bearers appear to equate that of natural timber but its service life is superior.