# An Optimization Model for a Desert Railway Route Scheme Based on Interval Number and TOPSIS
## 2 Research Methods
### Construction of the Evaluation Index System of Desert Railway Route Scheme  
#### 2.2.1 Analysis of the Evaluation Factors  
The alignment design of the desert railway is essentially limited by geological and environmental factors. The main hazard confronted by the railway in construction and operation is the wind-sand disturbance. Given the fragile ecological environment and  
Figure 1: The research framework. Railway alignment design in desert areas aims to select the best route scheme that is economically reasonable and technically feasible according to the transportation needs and natural conditions. The terrain, geology, hydrology, meteorology, and other natural conditions determine the difficulty of design and construction of railways, and then directly affect the selection of the line position [16; 17]. The sand hazard should be rationally avoided and controlled during the engineering design. Due to a large number of construction sites and the wide scope of railway construction, the scale of land exploitation has grown substantially. Specifically, in wind-sand zones, the proportion of bridges and tunnels for railways is trivial, resulting in more occupied land. The original surface can be readily damaged during railway construction to form a new sand source [18]. The proportion of bridges and tunnels is closely related to earthwork, land occupation, and capital investment. Increasing this ratio can reduce land occupation, route slope, and line length, which is conducive to improving railway transportation efficiency and ensuring operational safety. The setting of bridges avoids the cutting effect on the environment on both sides of the line and facilitates the migration and foraging of wild animals [19]. Some sections with serious quicksand can also adopt the design of replacing subgrade with bridges to prevent sand damage [20]. The railway in the curvy segment is more vulnerable to the threat of sand damage. Generally, desert railways should be provided with few curves, and curves with a small radius of curvature should be avoided as much as possible. The railway in the curve section is usually designed as an embankment with the outside facing the prevailing wind direction. Wind-blown sand hazard is the main geological hazard faced by the construction and operation of the desert railway. A railway is a strip-shaped three-dimensional space entity arranged on the ground. By rationally considering the characteristics of the gas-solid two-phase flow of the wind-blown sand, appropriate engineering considerations can block its movement. The main forms of the railway sand damage are track burial, subgrade erosion, and rail abrasion [21; 22]. The subgrade blocks a large amount of sand on the windward side of the railway, causing serious sand accumulation on the upper part of the track. The blocking effect strengthens with the increase in the angle between the line direction and the prevailing wind direction [23]. The local environment on both sides of the railway has a particularly prominent impact on wind-sand hazards [24]. Sand is the material basis of aeolian sand movement [25]. The more abundant the sand sources along the railway, the more likely it will suffer from wind-sand hazards. The sand control engineering of railways commonly refers to the engineering facilities set up in the sections vulnerable to sand damage, controlling the occurrence of wind erosion and altering the conditions of sand transportation. Further, sand prevention measures include engineering, vegetation, and chemical measures. Plant sand fixation in sections with good groundwater not only achieves the goal of permanent wind protection and sand stabilization, but also has a remarkably positive effect on the ecological environment along the railway. Environmental impact  
The construction and operation of the railway can inevitably affect the natural environment and ecological environment along the route.