A B S T R A C T
The spatial structure of urban regions has mainly been shaped by the progress in transportation and communication, and structural and transportation policies have been exerted in the development of metropolitan areas. The expansion of the suburban rail network impacts suburban urbanization and the change of residential locations. It changes the density and social and economic characteristics in the peripheral cities. The present research is descriptive-analytical, and its nature is practical. The information collection was a library, and with a morphological approach, it investigated the effects of the intercity rail network on the characteristics of the spatial organization of the population living around Karaj stations. This research analyzed 6190 census blocks (1996 and 2016) through Pearson correlation and kernel density in ArcMap software. The research results show that various demographic, social, and economic indicators strongly correlate with the distance from the railway network stations, and the kernel density of the above indicators also emphasizes the spatial proximity of the densities to the railway stations. By using the results of the research, it is possible to control the spatial organization of the population at the level of the metropolitan regions of Tehran and its post-suburban development, and by linking the cities and even the peripheral metropolises to Tehran, the pattern of migration and urbanization can be guided.
Extended Abstract
Introduction
In recent decades, the spatial structure of metropolises has experienced decentralization and integration. The spatial development of urban regions is defined as a process of external diffusion of functions from big to small cities. It depends on demographic, economic, infrastructural, and technological factors. Highway and rail networks have played a structural role in expanding peripheral cities and have been used as a powerful tool to advance spatial policies. Few studies have been conducted on the impact of the rail network on urban development, and dense Asian cities have little empirical evidence on the relationships between "proximity to urban rail stations" and "housing development." While progress in transportation has had an impact on the formation of the spatial structure of modern cities, reducing the cost of space, expanding suburbs, developing new centers, distributing population and activity, reducing the population of the central city, and improving spatial proximity and integration. In recent years, the rail network development program to connect new and existing cities has received more attention than in the past, and the rail network is directly effective on population density, employment, migration rates, the mix of land uses, land prices, and development investment. In the future, we will face a new system of rail access networks in its peripheral areas, which is different from the highway model. This study is mainly limited to the physical development, population density, immigration, and economic role of the rail network.
 
Methodology
The current article is applied and descriptive-analytical research. The method of data collection was documentary. The approach of this study was morphological. This article used spatial and statistical analysis. Analysis, including Cluster and Outlier Analysis, point density, kernel density, and Pearson correlation, have been used 10 km from the railway stations of Karaj to identify the intensity of population clustering and the spatial trend and correlation relationships. The study area is the population and housing census blocks of 2016 at a distance of 10 kilometers from the city's railway stations, and the trend of spatial development compared to 1996 has been investigated. Also, the correlation between the variable "minimum distance from the railway station" and other variables has been investigated, and the correlation has been calculated in the number of 6190 urban blocks. It is necessary to explain that the number of dependent variables in this analysis equals 33, including various indicators of employment, immigration, housing, literacy, etc.
 
Results and discussion
In the intercity rail network, the most significant traffic volume is allocated to Golshahr and Karaj stations in Karaj and Sadeghieh station in Tehran, respectively, and these stations have formed a chain of origin-destination. The creation of the railway network created a new process of development. During the operation period of the railway network (1996-2016), a high share of the population increase in Karaj occurred. The point density analysis of the population distribution in 1996 shows that the residential area of Karaj has a degree of density saturation compared to other areas. After 20 years, its density has continued. There is a significant relationship between the distance from the railway station and many social, economic, and physical indicators (34 variables), and 33 variables correlate with a significance level of 1%. With the expansion of the railway network, new centers have been formed both around Golshahr station and on the Fardis route. The population density has increased around the railway stations, and the indicators of population density, employment, and migration have been established around the stations. Rail access has played a role in facilitating the residence of retired households, relatively extended households, single households, and female heads of households around the stations and has contributed to housing ownership outside the central metropolis. In this range, the high density around the station indicates the high desire of households to be close to the intercity rail network, which means that the travel flows towards the central city have gathered more households around the rail access.
Conclusion
The expansion of the rail infrastructure offers a high capacity of travel in urban areas; considering the growing trend of the metropolitan area of Tehran, it is one of the physical infrastructures for the residence of immigrants. Improving and strengthening accessibility is effective in strengthening, balancing, and integrating the sub-centers of the metropolitan area. Reducing the daily travel time by using high-speed rail technologies expands the functional area of the metropolitan region of Tehran. It provides a connection with the surrounding metropolises, which has a positive effect on the balance and integration of future development.
 
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.