Why do subways always break down? Netizens: This month's full attendance bonus is gone again?!

Audit expert: Hu Qizhou

Associate Professor, School of Automation, Nanjing University of Science and Technology

"Dear passengers: There is an equipment failure on Beijing Subway Line 10 and the trains are running slowly. Please follow the instructions of the station staff. Passengers in a hurry are advised to choose other means of transportation. We apologize for the inconvenience!" Friends who commute by subway in Beijing will be familiar with this voice.

During the morning rush hour on March 13, the signal system of Metro Line 10 malfunctioned, stranding a large number of passengers. Official data from the Beijing Metro showed that the passenger flow on Line 10 that day decreased by about one-tenth compared to a week ago. The subway failure seriously affected public transportation order and the travel of office workers.

Source: Netizen @Doug Jinsi Goer

Not only in Beijing, but also in subway operations across the country, such as Chongqing, Nanjing, Shenzhen, Shanghai, Guangzhou and other places. According to statistics, the most frequent subway failure is signal failure. So, what is the subway signal system? Why does subway signal failure occur? Why do mobile phones often have no signal in the subway?

1What is the subway signal system?

The subway signal system is a system that directs and controls subway operations through modern automatic control and remote control technology, computer network technology, communication technology and other advanced technologies. Generally speaking, the signal system determines the direction of train travel by controlling the turnout . The turnout refers to the use of the motor next to the track to drive the rails to move to another set of rails to achieve the change of track direction.

Subway turnout

Source: Tianjin Metro

In addition, the signal system will also decide the running speed of the train based on the internal complex logic calculations and the time and location information of each train, and transmit the signal to the train control interface or the signal machine to provide guidance for the train driver. This may be a bit complicated, and you can simply understand it as: "The train driver controls the subway operation in the tunnel through various signals transmitted by the signal system." The most widely used signal system at present is the CBTC system , which refers to the use of advanced scheduling and organization technology to simultaneously control the operation of the train, and can shorten the waiting time of passengers and improve the transportation capacity of the subway line. The system has two operating modes: standard mode and backup mode . The standard mode is also the normal operating mode, which means that the train driver controls the train operation based on the information on the control interface. As shown in the figure below, the digital marking part 1 is the vehicle door information and platform door display, etc., which is used to provide the driver with the working status display of the door and platform door; the digital marking part 2 is the target running speed of the train; the digital marking part 3 is the signal recommended speed (yellow mark in the dial) and the emergency brake trigger speed (red mark in the dial), which is convenient for the driver to drive the train at the recommended speed and not exceed the emergency brake trigger speed.

Vehicle signal interface

Source: Li Chunming, Gao Chunbing, Yang Yifan. Research on the integration scheme of subway vehicle and signal system display screen[J]. Railway Communication and Signal, 2023, 59(2): 70-74.

Once the signal system fails, the CBTC system will start the backup mode, and the subway needs to rely on the ground signal machine beside the track to drive to the designated location for fault recovery. The ground signal is similar to the traffic lights on the road, including color light signal machines, hand signals, etc. At this time, the passengers in the car need to wait in a closed space for the fault to be repaired.

Subway signal

Source: Nanning Metro

2Why do subway signal failures occur?

Now that we know the role of subway signal systems, let's talk about why subway signal failures occur frequently. The CBTC signal system is a complex system that includes multiple subsystems. Its main subsystems include area controllers, onboard controllers, train automatic monitoring data information systems, and driver displays, etc. It involves a large number of devices, including not only train control equipment but also signal transmission equipment. Therefore, anything from a major onboard equipment/ground equipment failure to a minor signal transmission error may cause a signal system failure.

Generally speaking, signal failures are mainly divided into three categories: hardware failures, software failures, and communication failures. Hardware failures are mainly equipment failures, loose connectors, line failures, protection circuit failures, equipment aging, etc. For example, on the last day of 2019, Shenzhen Metro Line 4 was slow to run due to a contact network failure. On November 12, 2021, Shanghai Metro Line 1 was delayed due to a power supply equipment failure.

Source: Sina Weibo

Software failures include program running errors, interface errors, etc. For example, on October 16, 2018, four train lines of the Hong Kong MTR failed at the same time, restricting the travel of more than one million people and paralyzing traffic. The cause of the accident was the uncoordinated reset of the software counter of the signal system when switching between the old and new systems.

Source: China News Service

Communication failures mainly include network failures and signal interference. On October 8, 2012, a trainee mistakenly sent the words "Wang Peng your sister" to the display screen on the platform of Beijing Subway Line 5, affecting passengers' viewing of travel information.

Source: Internet

Therefore, as a key urban infrastructure, the subway still needs to strengthen safety risk analysis and improve emergency response capabilities to avoid failures to the greatest extent possible.

3Why is there often no mobile phone signal in the subway?

In addition to subway signal failure, another problem many people encounter on the subway is the lack of mobile phone signal. Although almost all lines have achieved signal coverage, some passengers still report that the signal is not very good and the network connection is intermittent. Why is this? The subway communication system is mainly divided into three parts: dedicated, civil, and police communications. Mobile phone signals belong to the scope of civil communications and are mobile communication networks built by the three major operators.

The quality of mobile communication networks is related to the network coverage of base stations, and the network coverage is often related to the number of base stations. Since mobile phone signals are microwaves, obstacles or rainy and snowy weather will block and attenuate some of the microwaves. Therefore, the quality of mobile phone signals is related to the network coverage of base stations, the distance from the base stations, the subway station environment, the weather, and so on.

Base Station

Source: Baidu Encyclopedia

Next, let's talk about the distribution and construction of the subway communication system. Due to the differences in the environment of different sections of the subway, the construction of the communication system also has certain differences. For example, for the control center, the antenna for transmitting signals is generally set up on an outdoor tower , or the way the antenna and base station cooperate is determined according to the coverage area; while the communication system of the platform usually sets the cable of the wireless network in the tunnel ; for the vehicle driving section , because there are a large number of signal coverage blind spots, leaky coaxial cables are usually used to make the signal coverage uniform . Although the subway communication system has a wide coverage area, due to the limited load capacity of the base station, the large traffic demand will still affect the mobile network signal to a certain extent. In addition, when the subway is running fast, it may just enter the range of a base station and then connect to the next base station range in a short time. The rapid change of different base stations will also make the passenger communication experience worse; of course, the signal coverage of some locations is weak, which cannot match the growth rate of user traffic demand... These will also cause the mobile phone signal to weaken.

This is the end of the answer to the subway signal failure and mobile phone signal failure. I hope you can stay calm next time you encounter similar situations.