Although vibration measurements are very useful and common in practice nowadays in many applications related to mechanical, civil, and aerospace engineering, the utilisation in railtrack infrastructure and facilities is not widely established. This chapter deals with the application of vibration measurements and finite element model updating to the assessment of ballasted railtrack sleepers, in particular with a void and pocket condition. It describes the concept of vibration measurements and the understanding into the dynamic behaviour of ballasted railtrack sleepers. It discusses briefly on the development of finite element model of in-situ sleeper and its updating. Then, the application to structural health monitoring of the rail-track sleepers is demonstrated. Dynamic load effects on the in-situ concrete sleepers in a railway track system are very significant and highly regarded in the viewpoint of structural engineers since the resonant excitation frequencies amplify the vibration magnitudes and cause cracking of railway concrete sleepers. New concept for the design criteria of railway concrete sleepers considers such resonant effects. As one of the main components of railway tracks, ballast interacts with the sleepers and influences the dynamic characteristics of railway sleepers, as well as the dynamic modulus of railtracks. The ballast support configurations are often changed by the effects of wheel load, breakage of gravel, or loss of confinement, which therefore creates voids and pockets underneath railway sleeper. This chapter presents a comparison between the experimental investigation on free vibration behaviour of an in-situ railway concrete sleeper with voids and pockets underneath, and the numerical prediction incorporating sleeper/ballast interaction. It is aimed at demonstrating the development and application of the vibration measurement and model updating to railway engineering practice. Using finite elements, Timoshenko-beam and spring elements were used in the in-situ railway concrete sleeper modelling, whereas the voids and pockets could be superiorly treated. This model had been proven its effectiveness for predicting the free vibration characteristics of insitu sleepers under different circumstances. In addition, the modal testing results have clearly exhibited that the simplified approach is ample to predict the natural vibrations of voided railway concrete sleepers. This study has led to the prediction and understanding of dynamic characteristics of railway concrete sleepers under various configurations of voids and pockets.