## 3 Network Failure Mechanism in Signaling System Failure Scenario
### Network Failure Mechanism  
Established studies mostly focused on low-frequency and high-loss types of man-made disturbances and natural disturbances, while less consideration was given to high-frequency low-loss types of technical disturbances. According to China's urban rail transit data statistics, signaling system failure accounts for 42.53% of technical disturbance events in urban rail transit [13]. On 27 September 2011, after a signal failure at Xinitiandi Station on Line 10 of the Shanghai Metro, the telephone occlusion method was used, resulting in a rear-end collision of the train and injuring 271 passengers [45]. All these data indicate that although the disturbance impact and damage level of signaling system failures can be easily predicted, they occur with high frequency, and when they occur, they must also be technically overhauled before resuming operation. Therefore, the problem of signaling systems affecting the stable operation of rail transit needs to be solved urgently. In this paper, the technical disturbance scenario of signaling system failure was used as the background to analyze the network failure mechanism of urban rail transit after the failure, and the evaluation of station importance is carried out on this basis. Interlocking system for urban rail transport refers to the mutual constraints established by technical means between signaling machines, turnouts, and approaches to ensure the safety of running and shunting operations in railway stations [46]. The equipment that realizes this relationship is called interlocking equipment, and according to the interlocking equipment, rail transit stations can be divided into interlocking stations, element control computer (ECC) stations, and non-centralized stations. Among these, the ECC uses the component control computer to extend the operation capability and control range of its signaling system. Its basic function is to send and issue control commands to external execution devices, receive dynamic information from these devices, and feed it back into the interlocking computer to complete the feedback process of information [47]. That is, it outputs commands and collects information to all non-centralized stations within the ECC boundary and sends the dynamic information received back to the interlocking station. The role of the interlocking station is to transmit and receive information from all ECC  
Figure 1: Urban rail transit network modeling method. stations within the interlocking boundary. The schematic diagram of an urban rail transit signaling interlocking system is shown in Figure 2.  
Based on the interlocking relationship of urban rail transit signaling systems, the network failure mechanism is defined. When signaling system failure occurs in urban rail transit, there are three scenarios. If the ECC station fails, it will cause the station and line within the control range of the ECC station (within the ECC boundary) to lose control of the outdoor executive equipment and stop receiving dynamic information, making the station and line within the ECC boundary fail. If the interlocking station fails, the station and line within the control range of the interlocking station (within the interlocking boundary) will not be able to receive and feed back the information, which means that the stations and lines within the entire interlocking boundary fail. If the non-centralized station fails, the station will not be able to communicate with the ECC station, which means that the station and its connected line fail. Based on the above analyses, we know that the signaling failure incident at Xintiandi Station of Shanghai Metro Line 10 in 2011 had a huge impact on network operation and passenger travel. Figure 3 provides an example of the network model before and after station attacks. Figure 3a,b depict the actual wiring diagram and the abstract network model, respectively. Figure 3c,d simulate two types of attacks resulting from signaling system failures.