Evolving Characteristics of High-Speed Railway Network Structure in Yangtze River Delta, China: the Perspective of Passenger Flows

China's high-speed railway (HSR) network is the largest planned network operating globally. Compared to other HSR networks, it has a unique and complicated structure. Multiple studies explored the supply side network structure of HSR services through train frequency data. However, due to data collection difficulties, there is limited research that studies the evolving characteristics of HSR development by using passenger flow data on the demand side network. This paper explores the evolving characteristics of the HSR network in the Yangtze River Delta (YRD), one of the most developed megaregions of China. HSR passenger flow data from the timeframe of 2014 to 2018 are used to examine the evolution of topological structure, hierarchical structure, and the spatial structure of the HSR network. Results indicate that the topological structure of the HSR network has been relatively stable, and has a roughly normal distribution. Although it is a relatively stable HSR network, a hierarchical structure exists, and there is a certain degree of variability among the third-tier cities. Based on the weighted degree centrality, and structural holes of passenger flow network in the YRD, Shanghai, Hangzhou, Nanjing, and Hefei, were identified. Correspondingly, four network communities were formed, and they varied across the evolutionary characteristics over the study period. The study proposes that in the planning of the HSR network, more focus should be given to connections between sub-regional nodes, in addition to connecting the core nodes. Additional relevant planning strategies are proposed based on the conclusions reached.