Full-scale fatigue performance evaluation of a novel frame-type ballastless slab track: Insights from the Istanbul airport metro project

In recent years, ballasted railway tracks have increasingly been replaced by ballastless slab track systems, including area- and frame-type designs, on critical infrastructure such as high-speed railways, tunnels, and bridges. These systems offer enhanced stability, faster and more comfortable travel, and reduced maintenance costs. Among these, frame-type slab tracks are more cost-effective to produce compared to area-type tracks. Therefore, this study evaluates the fatigue behavior of a newly developed frame-type ballastless slab track system installed on the Istanbul Airport metro line. A full-scale experimental setup was used, following an established testing protocol. While the system successfully satisfied fatigue performance criteria over three million cycles, with maximum railhead displacements below 1.1 mm, it failed to meet vertical static stiffness requirements due to variations in under-rail pad properties. Subsequent component-level tests revealed high stiffness heterogeneity among pads. These findings underscore the importance of quality control in pad production and suggest that current testing procedures should incorporate variable stiffness values to better simulate in-service behavior.