## 2 Methodology  
### Construction Method of Special Wireless Sensor Network for Railway Status Monitoring  
The existing railway status monitoring system is mainly based on air and space integration technology. It combines the traditional ground monitoring system, on-board monitoring system, and vehicle-ground communication system, and joins the space monitoring and transmission system to realize the real-time wide-area monitoring of railway system. Although the railway status monitoring system based on air and space integration technology provides the richest and most comprehensive field data, as it is a complete closed system, the failure of any link may lead to the deterioration of the overall operation state of the system, and even lead to traffic safety accidents. However, wireless sensor network technology has the characteristics of easy installation, maintenance, expansion, and networking. It is more suitable for the construction of special wireless sensor network for railway status monitoring and real-time monitoring of the operation status of the system. The monitoring objects of the special wireless sensor network for railway status monitoring are mainly railway infrastructure and its operating environment, which are all deployed along railway tracks, so they are approximately linear or banded. The main function of special wireless sensor network is to sense the service status of railway infrastructure and its operating environment and send monitoring data to the base station near the monitoring area. Therefore, the deployment location of wireless sensor nodes and infrastructure correspond to each other. Wireless sensor nodes have limited energy storage, and their communication energy consumption is mainly determined by the amount of monitoring data and its transmission distance. At the same time, the communication distance of wireless sensor nodes is limited, so it is not feasible for the sensor nodes in the monitoring area to communicate directly with the base station. Therefore, the method divides the whole monitoring area into several sub-monitoring areas, and deploys a sink node at the edge of each sub-monitoring area to receive the monitoring data of sub-monitoring area, while the data in all sub-monitoring areas is forwarded from sink node to base station. On the one hand, the method can ensure that the data perceived by each sensor node can be sent to the base station smoothly; on the other hand, it also greatly reduces the energy consumption of sensor nodes and improves the life cycle of special wireless sensor networks. The architecture of double-layer wireless sensor network for railway status monitoring system is shown in Figure 1.  
As shown in Figure 1, the special wireless sensor network for railway status monitoring is a double-layer structure, which consists of sub-network layer and transmission backbone network layer  
Figure 1: Architecture of double-layer wireless sensor network for railway status monitoring system. The sub-network layer consists of several sub-monitoring areas. The wireless transmission network in each sub-monitoring area consists of several wireless sensor nodes and one wireless sink node. The sink node is responsible for collecting the sensing data of all sensor nodes in the sub-areas. The backbone network layer consists of several wireless sink nodes and a base station. The sink node is responsible for forwarding the data collected from the subnet layer to the base station. The service status data of railway infrastructure and its operating environment is first perceived by wireless sensor nodes in the sub-network, then the sensor nodes in each sub-monitoring area send the sensing data to the corresponding sink node. Finally, the sink node sends all the sensing data to the base station, and then the base station sends it to the remote server for the diagnosis and prediction of the status of railway system.