## 3 Theoretical Analysis  
Since the outbreak of the global environmental crisis, green technology innovation has become an important issue in academia and politics. To balance the development of society, economy, and environment, green technology innovation has been listed as the goal of the Chinese national innovation strategy. To be specific, boosting the growth of green technology innovation will help protect the ecological environment and achieve social and economic advancement at the same time. According to the literature review, it is learned that high-speed rail, because of its rapid passenger transport capability, can directly cause the rapid flow of human capital, improve the effectiveness of labor allocation, and bring about the diffusion of knowledge, which will directly promote green technology innovation. Furthermore, it is known that a high-speed rail service produces a considerable influence on industrial agglomeration, which is also an extensive factor affecting green technology innovation. Hence, this article further analyzes the mediating role of industrial collaborative agglomeration between high-speed rail and green technology innovation based on the hypotheses of New Economic Geography and agglomeration economics (as shown in Figure 1). Figure 1: Mechanism diagram of high-speed rail, industrial collaborative agglomeration, and green technology innovation. As stated by New Economic Geography and location theory, the spatial change of industrial distribution is a matter of the location selection of enterprises. High-speed rail, as a representative of rapid transportation services, is a principal element that affects the location of enterprises. High-speed rail improves the accessibility of a city, optimizes the allocation of resources, speeds up the information flow, and reduces transaction costs and manufacturing costs, attracting producer services and manufacturing and producing a longitudinal and transverse economic relation resulting in a collaborative agglomeration of these two major industries [44]. More specifically, high-speed rail reduces the cost of commuting. It can not only promote the transfer of the labor force between cities [45; 46; 47; 48] but also induce the redistribution of productive resources such as capital information and technology between cities [16; 39; 49]. Generally, areas with intensive knowledge and production factors are preferred locations for enterprises. Through connecting to high-speed rail, regions gain location advantages such as fast information flow and optimized resource allocation. Simultaneously, the expansion of transportation caused by high-speed rail can also reduce trade costs [50]. The convenient transportation allows enterprises to locate in cities where labor costs or land prices are lower [51; 52]. In addition, based on agglomeration economy, this article claims that industrial collaborative agglomeration can stimulate green technology innovation through industrial linkage effects, scale effects, and knowledge spillover effects. Firstly, the collaborative agglomeration of manufacturing and producer services shows a strong correlation between the upstream and downstream of the industry [53], which can enhance the effectiveness of resource allocation and utilization, improve the skill level of the labour force, and thus support the innovation of green technology. Industrial collaborative agglomeration means a clear industrial distribution of labor. The specialized segmentation of labor, which means that the manufacturing industry provides a large number of skilled professional labor forces while the producer service industry provides advanced and professional elements such as technology and information services as well as diversified financial resources [54], can increase the effectiveness of resource allocation. Besides, due to the sharing of professional inputs, such as labor, technology, information, and capital, the complementary advantages will be highlighted and the utilization rate of resources will be improved [55].