# An Optimization Model for a Desert Railway Route Scheme Based on Interval Number and TOPSIS
## 3 Case Study  
### Project Overview  
The Hotan-Ruoqiang railway is a single-track railway with a design speed of 120 km/h connecting Hotan to Ruoqiang in Xinjiang, China. The total length of its trunk line is 825.5 km. This railway extends from west to east along the piedmont proluvial plain at the northern foot of the Kunlun Mountain and the southern fringe of the Taklimakan Desert. It is another convenient network for Xinjiang to connect with other external regions. The corresponding geographical location has been illustrated in Figure 2. The areas along the Hotan-Ruoqiang railway are characterized by drought, sparse vegetation, and fragile ecology. It is difficult for the railway to avoid all environmentally sensitive areas and wind-blown sand districts along the route. Therefore, the wind-sand hazards and ecological environment are the key factors restricting railway construction. Figure 2: Location of the Hotan–Ruoqiang railway. The main influencing factors of the alignment design in the Minfeng-Yuhu section of the Hotan-Ruoqiang railway are the Niya National Wetland Park and moving dunes. This park has been spread from the Kunlun Mountain and ends in the hinterland of the Taklimakan Desert, an important local environmental protection barrier. In order to lessen the damage of the railway construction to the sandy zones and the environment, accounting for the park scope, sand hazard distribution, and topographic condition, the railway engineer examined Schemes A-C (Figure 3). Scheme A: The route turns to the southeast from the starting point of the comparison and passes through the moving dunes on the eastern side of the Yuhu. It then goes southward along the western edge of the Salwuzek Township and turns west to cross a 9.3-kilometre super bridge over a serious section of moving sandy dunes in the Minfeng East. Finally, after crossing the highway G215, this railway reaches the endpoint of the comparison. Scheme A completely bypasses the Niya National Wetland Park, passing only 11.87 km of the wetland and oasis, with the best environmental benefits. However, the railway crossing the moving dunes is the longest, which leads to the highest project investment and sand control work among the alternatives. Scheme B: The route starts from the starting point of the comparison and crosses the wetland protected area of the Niya National Wetland Park to the south. After crossing the G215 expressway, the route runs southward parallel to the national road G315, partially passing through flowing sand dunes. At last, it turns west and crosses the Tullekanrik River to the end of the comparison. The length of Scheme B is the shortest, and the corresponding project investment is less than that of Scheme A. The length of the railway that passes through the wetland oasis and the wetland park is about 18.5 km and 7.53 km, respectively, which has apparent environmental damage. Scheme C: The route starts from the comparison starting point, passes through the wetland conservation area of the Niya National Wetland Park, and runs southward along the southern side of the expressway. The route then crosses through the moving dunes and the expressway under construction by bridges and turns south. Finally, it turns west and crosses the Tullekanrik River to the comparison endpoint. Scheme C exhibits the largest railway length, but the corresponding engineering investment and protection quantities are the least. The railway passes through 22.1 km of wetland oasis and 11.88 km of wetland park, causing great damage to the ecological environment. From the analyses given above, it can be seen that the three route schemes have their own advantages and disadvantages in alignment design, wind-sand hazard, environmental impact, and operation condition, and it is difficult to determine what is optimal. Since the  
Figure 3: Schematic representation of the railway scheme.