Under tie (sleeper) pads – A state of the art review

•Under tie pads have become commonly used worldwide over in the last 20 years.•Under tie pads increase contact area and reduce contact pressure at crosstie-ballast interface.•Under tie pads decrease track stiffness and track settlement.•Under tie pads increase rail and crosstie acceleration and displacement, and rail bending.•Under tie pads reduce ground-borne track vibration and track resonant frequency. Under tie pads (referred to as under sleeper pads outside the United States) are elastic components that deliver a conformal resilient layer between the crosstie (sleeper) and the ballast. Track designs for heavier and faster trains have resulted in a need for more complex systems to decrease required maintenance and improve system reliability and utilization. They have demonstrated success in delivering on these requirements and have been a topic of substantial research over the past twenty years, though no standard understanding of their performance and behavior has been achieved. With this in mind, this paper provides a comprehensive review of the state of under tie pad (UTP) research and use. It introduces UTP technology, examines the physical, mechanical, and chemical properties of UTPs as well as how material selection affects in-track performance and discusses the different uses of UTPs. Based on the literature reviewed, UTPs can provide a reduction in substructure stresses, settlements, ground-borne noise and vibration, crosstie bending demands, and increase in track lateral resistance. UTPs can also result in increased rail bending stresses and track superstructure displacements. The paper also provides recommendations for future research to fill knowledge gaps currently in the literature.