# An Optimization Model for a Desert Railway Route Scheme Based on Interval Number and TOPSIS
## 4 Discussion  
The evaluation and optimization of the railway alignment scheme is a multi-attribute decision-making problem. First, we should establish an indicator system for project evaluation, but there is no unified standard. There is also little information about comparing and choosing the schemes of the desert railway route. The influencing factors on the route selection are tightly related to the cost, benefit, technology, safety, and surrounding environment [42]. Due to the fragile ecological environment and frequent geological hazards in desert zones, the geographical environment is very sensitive to engineering construction. The railway cuts off the movement path of the wind-sand flow on the surface, thus intensifying the imposition of sand damage and the destruction of the natural environment. Due to the long construction period and many crossing areas for the railway, the unreasonable route scheme will reduce its traffic efficiency, even leading to environmental and social problems. Once the environment in the desert area is destroyed, it is very difficult and expensive to recover [43]. The construction of the evaluation index system of the alignment scheme should not only consider the railway construction itself, but also focus on the ecological environment factors and sand hazards. Therefore, 18 indicators from the alignment design, blown sand hazard, environmental impact and operation condition are selected to establish the evaluation index system for railway alignment schemes in desert zones. The control factors of the railway route in the gale environment and the Gobi sandstorm environment are different from those in the desert areas. The index system developed in this paper has certain limitations. In practical application, the evaluation index system of the route scheme can be adjusted according to the characteristics of these sandy areas. In the current exploration, the subjective and objective weight methods are appropriately employed to determine the comprehensive weight of the evaluation index. A slight change in the index weight can have a remarkable impact on the evaluation results of the scheme. Because there are some qualitative indicators in the evaluation index system, it is difficult to evaluate the weight by employing the objective weighting method. At present, the analytic hierarchy process is commonly utilized to evaluate the index weight in the comparison and selection of railway route schemes; however, the calculation results are noticeably disturbed by human factors. The maximum deviation method determines the objective weight of the indicator based on the variance of the original data, which reflects the importance of the indicator from the data itself. Using interval numbers to quantify qualitative indicators also makes it more convenient to obtain objective weights. The comprehensive weight not only reflects the objective information of the decision-making, but also reduces the influence of subjective factors on the decision-making results. The optimal design of the alignment scheme for the railway is an important measure to reduce the environmental impact of railway engineering and wind-sand hazards in desert areas. Currently, the comprehensive evaluation of railway schemes often discusses qualitative and quantitative indicators separately. Through the calculation of quantitative indicators, combined with the qualitative analysis of qualitative indicators by experts, comprehensive evaluation results are achieved. These approaches do not realize the unified calculation of qualitative and quantitative indicators. The tedious calculation and complicated model reduce the efficiency and accuracy of railway scheme selection. Linguistic fuzzy numbers have some uncertainties. The quantification of the qualitative indicators through accurate numbers may lead to an incomplete expression of expert opinions, and ultimately the evaluation results do not conform to the actual situation of the project.