A B S T R A C T
Today, with the increasing population and the rising costs of developing road networks for personal vehicle use and their negative consequences, expanding public transportation systems such as railways is considered a fundamental solution for large cities. The purpose of this study is to determine the optimal shortest routes based on ecological and socio-economic criteria for the Gorgan–Mashhad railway within Golestan Province and to evaluate the positive and negative impacts of this project to select the route with the least environmental impact. In this research, multi-criteria decision-making techniques and the least-cost path algorithm were used to determine the optimal route for the Gorgan–Mashhad railway. First, influential parameters in route selection were identified, standardized, and weighted. Then, a cumulative cost map was produced in Idrisi software, and optimal routes were determined using the least-cost path algorithm. Finally, the routes were compared and evaluated.The routes obtained using the shortest path algorithm were as follows: Route (1) with 197 km, Route (2) with 177 km, and Route (3) with 164 km. Evaluation results for Route 1 indicate an average minimum distance of 308 meters from rivers, 13,505 meters from protected areas, 2,552 meters from residential areas, 6,176 meters from faults, an average elevation of 259 meters, and an average slope of 8.7%. Therefore, despite being longer, Route 1 causes less environmental damage. In the long term, Route 1 is more cost-effective and safer in terms of environmental costs. The results show that although the routing process is complex, suitable routes designed according to environmental principles can be achieved using GIS, multi-criteria evaluation methods, and the least-cost path algorithm.
Extended Abstract
Introduction
Nowadays, with the growing population and the increasing costs of expanding road networks for private vehicle use, along with its negative consequences, the development of public transportation systems such as railways is considered a fundamental solution for large cities. On the other hand, developing transportation systems without proper planning and management can lead to damage to the natural environment. Accordingly, to reduce damage to environmental resources and achieve sustainable development goals, environmental characteristics must be incorporated into projects related to linear infrastructure, such as railway lines. In traditional routing methods, technical and engineering specifications are prioritized, while environmental characteristics receive less attention. Geographic Information Systems (GIS) are among the efficient and effective tools for assisting experts in designing linear infrastructure. One of the most beneficial applications of these tools is determining the least costly and least impactful route to connect two locations, considering environmental characteristics relevant to the subject and location. The project for developing the national railway route in Golestan Province towards the east (Khorasan Razavi Province) has been under consideration for some time. In this process, a route exists in the provincial maps that can be compared with the results of the current study. It is evident that implementing this project holds significant importance and will impact the province's development in environmental, economic, and social aspects.
Methodology
Overall, the methodology of this research is divided into three main steps:

Defining the boundaries of the study area and obtaining base maps and data.
Identifying the shortest environmentally-friendly proposed routes using land-use maps and other relevant maps for routing between the city of Gorgan and the easternmost point of Golestan Province, utilizing the Idrisi Terrset 2020 software.
Evaluating the environmental impacts of the proposed routes and selecting the final route.

Considering the objective of this research, the routing begins at the starting point, which is the Gorgan railway station, and ends at the easternmost point of Golestan Province. Therefore, the boundary of the study area is limited to Golestan Province. In this study, multi-criteria decision-making techniques and the least-cost path algorithm were used to determine the optimal route for the Gorgan-Mashhad railway. First, after library research and reviewing various sources, a list of necessary parameters for railway routing was compiled. Based on the input of the thesis advisory committee, 35 parameters were selected for the Gorgan-Mashhad railway routing. After standardizing and weighting these parameters, a cumulative cost map was generated in the Idrisi software environment. The least-cost path algorithm was then used to identify the optimal routes. Finally, the routes were compared and evaluated.
 
Results and discussion
In this study, the Weighted Linear Combination (WLC) method and the Least-Cost Path Algorithm (LCPA) were used to integrate the layers. In the first stage, land suitability for constructing this infrastructure was evaluated using appropriate Weighted Linear Combination techniques. Then, the optimal route between Gorgan and Mashhad was determined on the cost or friction surface using the optimal routing method (LCPA).
It is important to note that the criteria obtained from Multi-Criteria Evaluation (MCE) must be dimensionless and scale-free. Therefore, in this research, fuzzy logic and Boolean methods were applied to standardize the criteria. Finally, in the Idrisi TerrSet 2020 environment, fuzzy maps, constraints, and weighted criteria were combined using the Weighted Linear Combination method. This process produced a multi-criteria evaluation map, which was then integrated with the regional suitability map to obtain the final assessment.
Subsequently, using the cost or friction map, the cumulative cost map, and the origin and destination points in Idrisi TerrSet 2020, two optimal routes were identified. Additionally, the Planning and Management Organization provided a route based on their preliminary studies. After designing the routes, they were evaluated to select the one that caused the least environmental damage as the optimal route. In fact, selecting the optimal route means choosing the most cost-effective path in terms of environmental and economic considerations.
The final route lengths obtained using the shortest path algorithm are as follows: Route 1 is 197 kilometers, Route 2 is 177 kilometers, and Route 3 is 164 kilometers.  Additionally, key criteria for comparing and evaluating the routes were selected based on the review of various studies. These criteria include: average slope, average elevation, average distance from faults, average distance from residential areas, average distance from protected areas, and average distance from rivers.
The evaluation results for Route 1 indicate that the average minimum distance from rivers is 308 meters, the distance from protected areas is 13,505 meters, the average distance from residential areas is 2,552 meters, the distance from faults is 6,176 meters, the average elevation is 259 meters, and the average slope is 8.7%.
Based on the obtained results, although Route 1 is longer than the other routes, it maintains an appropriate buffer from environmentally sensitive factors. From an economic perspective, despite being farther from residential areas compared to other routes, it still offers reasonable accessibility and is not significantly distant from populated areas. Moreover, Route 1 has a lower average elevation and slope percentage, which leads to reduced excavation and embankment costs. Additionally, by maintaining greater distances from faults and protected areas, this route enhances railway safety and helps preserve the ecosystem and wildlife habitats in the region.
 
 
 
Conclusion
A path that takes into account all influencing factors, such as construction costs, safety, and environmental issues, is referred to as a low-cost path. The first step in constructing any new route is selecting an optimal path. Designing the optimal railway route is a complex task that requires extensive data analysis and parameter evaluation, depending on the project's length. The results showed that using GIS, multi-criteria evaluation methods, and LCPA plays a key role in addressing various environmental dimensions.
In this study, influential environmental criteria were identified, and using the capabilities of GIS and LCPA, the optimal routes were determined. The designed routes were compared and evaluated in terms of adherence to distance constraints from relative obstacles, such as rivers, cities, villages, protected areas, and other environmentally sensitive regions. Ultimately, Path 1 was selected as the final route. Although Path 1 was longer, it is likely to cause fewer environmental damages and long-term impacts on the ecosystem and human health in the surrounding areas compared to the other two paths.
 
Funding
There is no funding support.
 
Authors’ Contribution
Authors contributed equally to the conceptualization and writing of the article. All of the authors approved thecontent of the manuscript and agreed on all aspects of the work declaration of competing interest none.
 
Conflict of Interest
Authors declared no conflict of interest.
 
Acknowledgments
 We are grateful to all the scientific consultants of this paper.