## 2 Literature Review
A generic operational planning process [27; 28] in railway transportation is illustrated in Figure 1, which generally follows a hierarchical top-down approach, from strategic network design and line planning, tactic timetabling, rolling stock planning and crew scheduling, to operational re-scheduling [25; 29; 30; 31]. In most cases, studies explore the integration of line planning and timetabling or timetabling and rolling stock scheduling [32; 33; 34; 35]. Line planning is often scheduled before rolling stock circulation planning, which may lead to the infeasibility of the following operation plan and increase the complexity of the operation plan [34]. If such systems are subject to limitations of resources, a dynamic transfer of resources from one line to another throughout the planning horizon should also be considered. This study focuses on the integrity of line planning and rolling stock scheduling in the form of resource constraints. The line planning problem is the first planning stage after network design in the public transportation system [36; 37], which is an important basis for formulating the train timetable and resource circulation scheme. Thus, it is essential to study the new line planning approach with fluctuating passenger flow demand [6]. In this paper, we proposed a line planning approach in response to the space non-equilibrium and time-dependent demand. As stated by [PERSON] [6], the line planning problem with  
Figure 1: Illustration of railway planning procedures. fluctuating demand comes inherently with resource constraints to ensure the scheme's feasibility. Different from [PERSON] [6], we think that resources may be scarce during peak periods, and discussed the impact of the number of resources on the line planning scheme. We are motivated by the lack of line planning approaches to handle high variations and fluctuations of demand in both time and space. The flexible composition mode is an active method that can cope with complex passenger demand changes by making full use of the transport capacity of each rolling stock [4; 38]. We aim to discuss multi-period line planning, recognizing the connection between period-coupling resource constraints in multi-period planning and the integration of line planning with resources that are associated with later stages of planning. We will mainly complete the following three tasks: (i) Proposing a multi-period planning approach in response to high levels of fluctuation and variation in demand. (ii) Integrating the multi-period strategic-level line planning decision with resource constraints that ensure the availability and allocation of operational resources throughout the periods. (iii) Applying the flexible composition mode to the LPP. This study focuses on developing a novel multi-period line planning approach that considers a changing demand and flexible composition mode. The optimization method is developed to solve the line planning problem with a profit-oriented approach considering the transfer of carriage resources during a finite length planning horizon of multiple periods. By this method, it is expected to match the passenger demand with the scheduled transportation capacity, further improve operational flexibility, and extract gains in efficiency through flexibly determining the number of train formations and formulating corresponding operation plans according to the demand in different periods.