## 2 Literature Review  
High-speed rail (HSR) systems have been widely used in China especially in the eastern and southern areas. Due to the special geological conditions and the high standards of HSR systems, tunnels are often necessary for a long, high-speed railway line. The HSR tunnel proves to be quite complicated [5] because of the following reasons:  
* The construction of a railway tunnel requires a higher number of construction processes compared to normal railway construction. Before a railway's construction, a tunnel should be excavated and supported gradually. These excavation and support processes are likely to be much more complex and time-consuming than subsequent railway construction. * Compared to normal railway construction, different machines and equipment are used which may lead to more difficult construction management. * After construction, the operation of a railway tunnel will influence the tunnel itself in various ways. * A railway tunnel is often very costly and should be designed for long life. Therefore, a tunnel's maintenance and modifications can also be very costly. * The operation of a railway tunnel needs more personnel and equipment. Due to the above issues, a lot of money is required to invest in a railway tunnel. Railway tunnel construction can have a significant impact on the environment. Therefore, costs and the environmental impact of railway tunnels require further study based on the currently available analysis of normal railways. Various approaches are applied to estimate the costs of a railway tunnel system. Generally, these methods can be classified into two categories. One is to perform case studies on given railway lines using a bottom-up method. The other is to take an average system using a top-down method. [PERSON] et al. [6] applied the case-study method to evaluate the full costs of a high-speed rail system in California including infrastructure, fleet capital and operating costs, time, and social costs (e.g., noise, pollution, and accidents). [PERSON] [7] carried out a similar case study for the high-speed railway system between Osaka and Tokyo using the Japanese Shinkansen technology in California. [PERSON] [8] assessed the costs of the construction and operation of a new high-speed railway system. Generally, the case study approach can be useful for the cost evaluation of high-speed railway systems. However, in this case, it is confined to a single specific project and is not able to provide a very general discussion of the costs. In contrast, the second approach, the top-down method, establishes a much more general model for the estimation of high-speed railway costs. There are two different methods for this approach. The first is the life-cycle cost (LCC) method. LCC is applied only to individual parts of a whole system and evaluates the overall costs throughout an entire life cycle of a high-speed railway system. LCC analysis is an economic assessment that accounts for all possible significant costs over the design life of a railway tunnel [9]. [PERSON] [10] developed LCC models for the rail and ballast of a railway system which is used for the initial decision-making process of the project. [PERSON] et al. [11] have performed a similar analysis to optimise ballast tamping and renewal to reduce the LCC of the ballast. The second is the RAMS (reliability, availability, maintainability, and safety) analysis which is useful from a long-term point of view. This method mainly concentrates on the maintenance management of railway systems and operation and maintenance costs throughout the overall life of the design life [12]. [PERSON] [13] developed a method for effective decision making during the maintenance process of a railway system using both LCC and RAMS analysis. Based on the above analysis, LCC is a more comprehensive cost evaluation method because it can account for each individual portion of costs throughout the overall life of the design.