Railway vehicles running on track structures constitute an interactive and inseparable large-scale dynamics system. For a long time, studies on railway vehicle dynamics and on track structure vibration were carried out separately due to the complexity of wheel-rail nonlinear systems and the classification of disciplines, which consequently results in the classical theory system of separate vehicle dynamics and track dynamics. However, the dramatic increase of operating speed and hauling mass of modern railway transportation intensifies dynamic interactions between vehicles and tracks, and such complicated vehicle-track interaction problems fail to be handled by the classical subsystem dynamics theory. Therefore, it is necessary to break through the subsystem research framework and comprehensively investigate the vehicle-track interaction mechanism, so as to achieve the optimal dynamic performance design of the whole system. For this purpose, the vehicle-track coupled dynamics theory system is developed on the basis of absorbing the theoretical achievements of vehicle dynamics and track dynamics, which treats the vehicle subsystem and the track subsystem as a large integrated system by establishing a wheel-rail spatial dynamic coupling model. The vehicle-track coupled dynamics problems can be divided into three aspects: The vertical, lateral and longitudinal coupled dynamics. Generally, the vehicle-track vertical coupled dynamics mainly studies the train-track vertical interaction problem. The vehicle-track lateral coupled dynamics focuses on the stability and safety of vehicle lateral movement on the elastic-damping track structure. In recent years, with the development of long marshalling heavy-haul trains, especially running in the complex environment of long ramps in mountainous areas, the longitudinal impulse of vehicles in a train and its influence on the track system are aggravated, bringing prominent safety problems of the train and track system, which is exactly the mission of longitudinal coupled dynamics research. In view of these three aspects, the vehicle-track coupled dynamics model has undergone the development process from simple to complex, including the unified vertical coupled model, the vertical-lateral spatial coupled model, and the vertical-lateral-longitudinal three-dimensional coupled model. In this paper, the theory system of vehicle-track coupled dynamics will be briefly introduced, including the academic rationale and its formation process. Most of the emphasis is laid on the construction of the unified model of vehicle-track vertical coupled dynamics (namely the fundamental model), and the vehicle-track spatial coupled dynamics model and its recent development. Based on the theory and its simulation platform, an optimal matching design principle of the vehicle and track dynamic performance is proposed, providing a scientific solution for high safety and high stability design of the modern rail transportation system. The vehicle-track coupled dynamics theory ends the long history of separate investigations on vehicle dynamics and track dynamics, and opens up a new research field of railway large-scale system dynamics. As a new theory system, it is recognized worldwide that the vehicle-track coupled dynamics has become the fundamental method for research on railway engineering dynamics after nearly 30 years of development. This theory has been widely applied to railway engineering practice and has successfully solved a series of key dynamics problems in tremendous railway engineering projects. For this, some practical engineering projects are selected to illustrate the application of the theory, including the investigations on the selection of the plane and vertical profiles design scheme for a Chinese high-speed railway line, the key parameters design for a high-speed railway with a design speed of 400 km/h, the design of rail asymmetrical grinding profiles for a heavy-haul railway curved line, the safety control of a long heavy-haul train under crush conditions, and the vibration reduction and optimization design of floating slab tracks for metro railways. It is demonstrated that the vehicletrack coupled dynamics theory and its applications strongly support the remarkable development of Chinese railways.
