## 2 Literature Review
### The Optimization Modeling Method for Train Operation Diagrams  
Regarding direct modeling, [PERSON] [17] created a model with the minimization of weighted travel time and maintenance operation time as the objectives, taking the train arrival and departure times as integer decision variables. [PERSON] [18] aimed to minimize the total waiting time of passengers, setting the train waiting time as a 0-1 variable and coupling it with the number of inbound passengers within the same time interval to achieve the optimization of the operation diagram for coordinated passenger flow control. In terms of spatio-temporal network modeling, considering the two-dimensional characteristics of the train operation diagram, a two-dimensional spatio-temporal network is typically employed for modeling. The safety intervals are transformed into incompatible constraints, and the objective function is transformed into the minimization of network costs, thereby converting this problem into the shortest path problem, and the decision variables are also transformed from integer variables to 0-1 variables. [PERSON] et al. [19] constructed a spatio-temporal network diagram to depict the spatio-temporal resource occupation relationship of trains and ultimately summarized the solution of the train operation diagram optimization problem as the shortest path problem of trains in the spatio-temporal network.  
After reviewing the current research on high-speed rail express delivery both domestically and internationally, we can summarize as follows:  
1. Scholars have defined and categorized high-speed rail express delivery from multiple perspectives, verifying its feasibility and laying a solid foundation for further studies.
2. Research has deeply explored the competitive and cooperative relationships between high-speed rail express delivery and traditional transportation modes such as road and air. Analyses of its competitiveness and cooperative strategies among different transportation modes provide new insights for the sustainable development of high-speed rail express delivery.
3. While international studies are limited, domestic scholars have conducted extensive research on predicting high-speed rail express volumes and developing transport organization models. However, these studies often focus on operational plans and timetable formulation, lacking in-depth discussion on dedicated high-speed rail express services. Additionally, optimization models generally consider the perspectives of operators and shippers, without fully addressing the impact on passenger transport.
4. Modeling approaches and methods for train timetable optimization are well developed, including direct modeling and spatio-temporal network methods. Research has extended to coordinated optimization with stop plans and passenger and freight transport, providing theoretical and practical support for further studies.  
Based on existing research, this paper will consider the impact of high-speed rail express delivery on passenger transport capacity. It will adopt a dual perspective of both passenger and freight transport, comprehensively considering four transport organization models to conduct a more thorough optimization study.