Impacts of high-speed rail lines on the city network in China

China's large-scale HSR network has a significant influence on both accessibility and connectivity, but little attention has been paid to its impacts on connectivity. Changes in connectivity largely affect the external relations among cities, thus influencing the structure of the city network. This paper examines the impacts of China's HSR network on the overall connectivity and nodal centrality of the city network, as evaluated by passenger trains from 2003 to 2014. This paper employs the graph index, average path length, clustering coefficient and three indicators of centrality, namely weighted degree centrality (WDC), weighted closeness centrality (WCC) and weighted betweenness centrality (WBC). The results show that the HSR network largely increased overall connectivity according to the increasing Beta index and clustering coefficient, and decreasing average path length. The HSR lines also increased the average centrality of the city network and the inequality of nodal centrality according to the WCC indicator, but they decreased the inequality of nodal centrality with regard to the WDC and WBC indicators. Meanwhile, the growing HSR network led to the centrality tended to intensify in large cities in terms of the WCC indicator, but intensify in small cities according to the WDC and WBC indicators. Spatially, the cities with HSR stations, population over 3 million and GDP over 100 billion RMB saw higher increases in centrality than the others; the cities with high values of WDC and WCC, especially the top 20 cities, tended to concentrate in the populous areas with well-developed economies (e.g., Yangtze River Delta), whereas the cities with high WBC values in 2014 were coupled with the hub cities of regional railway management administration.