## 6 The Network Robustness and Optimization of Two Networks
### Network Optimization  
Six commonly used optimization strategies were adopted to simulate and analyze the growth of passenger transportation and freight transportation network in the New Western Land-sea Corridor in four years, and the results are shown in Figures 9 and 10. Figure 8: Distribution of key network degree, cumulative degree and disturbance effect. The passenger network has a strong network expansion ability. In the four-year period, strategy 6 was the best optimization strategy and strategy 1 had the second-best  
Figure 10: Changes in average path length of freight network growth over four years under different strategies. Figure 9: Changes in average path length of passenger network growth over four years under different strategies. \begin{table}
\begin{tabular}{c c c} \hline \hline
**D Value Interval of Coupling** & **Coordination Level** & **Degree of Coupling** \\
**Coordination Degree** & **Coordination** \\ \hline (0.0–0.1) & 1 & Extreme disorder \\ [0.1–0.2) & 2 & Serious disorder \\ [0.2–0.3) & 3 & Moderate disorder \\ [0.3–0.4) & 4 & Mild disorder \\ [0.4–0.5) & 5 & Borderline disorder \\ [0.5–0.6) & 6 & Barely coordination \\ [0.6–0.7) & 7 & Primary coordination \\ [0.7–0.8) & 8 & Betweenness coordinate \\ [0.8–0.9) & 9 & Good coordination \\ [0.9–1.0) & 10 & Great coordination \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Classification standard of coupling coordination degree. Figure 11: Coupling coordination degree of passenger and freight transport in four-year period, (**a**) Passenger and freight coupling coordination in 2000, (**b**) Passenger and freight coupling coordination in 2006, (**c**) Passenger and freight coupling coordination in 2012, (**d**) Passenger and freight coupling coordination in 2018. In 2000, the economic level of the western region was low, and the transportation network was in the stage of infrastructure construction. As the industrial development in Southwest China gradually shifted from resource-based development to resource-based processing, freight transportation reduced its dependence on the railway. With the adjustment of industrial structure, the railway has changed the thought of \"attaching importance to freight and neglecting passenger transport\" for a long time to vigorously develop passenger transport. The level of coupling coordination between the passenger transport and freight transport was relatively low. Most cities were in mild or moderate disorder, and a few areas, such as Xi'an, Kunming, Liupanshui, and Guiyang were on the critical point of disorder. Chongqing and Chengdu were in a high coordination position. In 2006, the economy grew slowly, the main road of the transportation network was basically formed, and the population increased slightly. The coupling coordination level of the passenger transport and freight transport was improved, mainly reflected in Guizhou province, Guangxi province, and Inner Mongolia autonomous prefecture. The coupling coordination degree of Baotou, Qujing, Southwest Guizhou, South Guizhou, Nanning, and other regions gradually increased, which belongs to the type of near unbalanced. In addition, the level of coupling coordination in Chengdu has decreased, belonging to good coordination. In 2012, the economy of the western region grew rapidly, but the construction of the transportation network and population growth were slow. The freight capacity of the railway has been greatly improved while passenger transport has not achieved breakthrough development, which makes the coupling and coordination degree of the two declines, but Chongqing is still in high-quality coordination. In addition to the Xi'an coupling coordination degree rise, the Kunming, Nanning, Guiyang, and Liupanshui region coupling coordination level was in stable condition; therefore, it still belongs to the type on the verge of disorder.