## 2 Literature Review and Research Hypothesis  
Industrial agglomeration is defined as a state of industrial concentration in a specific space or region, which itself contains the concept of space. The research on industrial agglomeration and its influencing factors mainly focuses on three perspectives. First, agglomeration economics theory divides economies of scale in production into internal and external economies [10]. Internal economies of scale come from inside enterprises, while external economies of scale come from the spatial aggregation of similar enterprises. Spatial agglomeration can reduce the risk of labor shortage at enterprises, and the spillover effect in industrial agglomeration can enable enterprises to obtain extra income that is higher than that obtainable in non-agglomeration. The more enterprises in the same industry that gather in a certain place, the more favorable it is for the enterprise to obtain the required factors of production (e.g., labor force, capital, energy, and transportation), and the more efficient the enterprise's production. Second, new economic geography theory treats the differences in economic geography between two regions as a kind of influencing factor on industrial agglomeration [11; 12], one of which spawns the initial industrial agglomeration before affecting it through the increasing return effects of the new economic geography factors. According to new economic geography, the endogenous forces of economic systems eventually lead to the evolution and differentiation of regions, resulting in a core-periphery structure. In this structure, factors such as geography and resources give rise to subtle differences between the previously comparable economies of both regions. In a cyclic accumulation of causality, the core region exports goods to the periphery while satisfying local demand, creating far greater market demand than the periphery [13; 14]. This significantly facilitates the region's population absorption, capital accumulation, knowledge creation, enterprise reception, and even industrial relocation, boosting the core region's agglomeration superiority. Third, endogenous growth theory takes knowledge spillover as the main explanation for agglomeration [15]. Knowledge spillover is a communication process that occurs through interaction, communication, and exchange between individuals, enterprises, and even different subjects at the regional level, including the free flow and interaction of knowledge and talent in different regions [16; 17] and the exchange and cooperation between enterprises and R&D institutions in different regions [18]. In recent years, the high-speed rail has shortened the commuting time between cities within urban agglomerations, reduced the cost of face-to-face communication between professional and technical personnel from different cities, and expanded the spread of knowledge and technology. Frequentintercity high-speed rail trips have also increased the innovative interaction between cities and jointly promoted knowledge spillover. Relevant empirical study results also prove that knowledge spillover and the transportation network have complementary effects. Intercity high-speed rail allows for more highly skilled workers to benefit from face-to-face interaction, improves the output efficiency of knowledge, and promotes industrial agglomeration [19]. New economic geography takes the transportation cost as the most important factor influencing industrial aggregation. The improved transportation integration in urban agglomerations accelerates industrial agglomeration. Owing to the multitude of transportation modes making up the network layout of urban agglomerations' comprehensive three-dimensional transportation network system, transportation and time costs can be greatly reduced [20; 21], thereby deepening inter-regional openness and dissolving market segmentation.