The cause of the accident was revealed! Why did the subway suddenly break down?

Audit expert: Hu Qizhou

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

I believe many people saw a piece of news last night: some carriages on the Beijing Metro Changping Line were uncoupled!

Source: Toutiao

Beijing Metro announced on its official Weibo account that the last two carriages of the train were separated from the front carriages at the Xi'erqi to Life Science Park section. Subway personnel, firefighters and emergency personnel later arrived at the scene and evacuated passengers to Xi'erqi Station.

Source: Weibo screenshot

On the 15th, @北京交通字幕下载 released a message that a total of 515 people were sent to the hospital for examination, including 102 people with fractures, and no one died. The cause of the accident was that the track slipped in the snowy weather, causing the signal of the front car to degrade, and the emergency brake stopped. The rear car was located in a downhill section, and the snowy weather caused the train to slide, and the brakes were not effectively applied, resulting in a rear-end collision with the front car.

Source | Beijing Daily

Many netizens were shocked when they saw the news, because this is the first time that a similar accident has occurred in the Beijing subway in recent years. But in fact, the operation and maintenance of urban heavy rail systems have always been a common problem worldwide. Rail transit systems such as the London Underground and the New York Subway are facing various maintenance and loss problems. The Mexico City Metro Line 7 also had a car disengagement accident during operation on January 15 this year.

Source: Toutiao

We still need to wait for official follow-up reports on the specific details of this traffic accident. However, we can see from the pictures posted on the Internet that the fracture occurred at the connection between the carriages, which is also a concentrated area where people gather and rely on during the subway rush hour, so we have to talk about what happened in this place.

1

The "Swing Together" throughway

Many people often face the situation of being stuck in the middle of the carriage when encountering surging crowds during rush hour, so they will automatically or passively move to the connection between the two carriages; some people, because the journey is long, will choose to lean on the connection and "sway" along with the moving subway.

But this location is really dangerous!

Source: Shanghai Metro

The connection between subway cars that we are familiar with has a high-sounding scientific name, called "through passage". It can play a safety protection role, providing passengers with a safe and reliable passage, while also playing the role of sound insulation, dust prevention and rain protection.

Source: Wuhan Metro

During the running of the train, the movement between the carriages is relative, which causes the connection to shake greatly, and it is easy for passengers to lose their balance or fall. There are usually no pull rings for passengers to hold at the connection, only handrails. Under the action of centrifugal force, it is difficult for passengers standing here to control their center of gravity when the train enters a curve or an emergency brake occurs.

In addition, there are folding windshields at the connection of the carriages, which play a role in turning and buffering. When the train turns or goes up and downhill, the baffles will be squeezed and deformed. The originally stretched parts may be compressed at any time . If children play on the windshield or passengers accidentally put their hands on it, they are easily pinched.

Therefore, in many subway passages, there are clear slogans such as "No leaning" or "No holding" printed on them.

Source: Guangzhou Metro

2

What hooks do subways use?

Has anyone ever looked out the window at the connection between carriages when they were bored on the train? On ordinary "green trains", there is a set of couplers that seem to be trembling when in motion and ready to detach at any time - the James coupler.

Source: NetEase

It consists of three parts: the hook head, the hook body and the hook tail. When the locomotive needs to be connected, the two hooks get close to each other, and the hook generates an extrusion force from the inclined surface to squeeze the spring hook tongue open, forming a handshake of the hook. At this time, the rear spring of the hook will rebound, the lock pin will fall, and the hook will not be unhooked.

Source: Manufacturing Principle

However, the Jane's coupler also has limitations. It is only suitable for passenger and freight trains on conventional railways. For other types of railway transportation such as high-speed railways, subways, light rails, etc., the Jane's coupler cannot meet their connection requirements and safety standards.

In the subway industry, the three main types of couplers used are "=, +, -".

The "=" fully automatic coupler is located in front of the train head and is used for coupling between electric passenger cars or between electric passenger cars and engineering vehicles. It can realize the automatic connection of mechanical, electrical and gas circuits. On the train, the driver can directly control the coupling and uncoupling of the fully automatic coupler, which can make the coupling of the two trains more efficient and reliable, and provide strong guarantee for the safe operation of the train.

Fully automatic coupler source: Foshan Metro

The “+” semi-automatic coupler is located at the connection between the two middle carriages of the 6-carriage car, below the connection between the carriages. It drives the rear carriage to move forward steadily. It looks almost exactly the same as the fully automatic coupler, but because uncoupling and coupling operations are not performed frequently, they are usually disassembled for inspection once every two years. Therefore, it does not have an automatic uncoupling function, only an automatic coupling function.

Semi-automatic coupler source: Foshan Metro

Except for the two middle carriages, the semi-permanent traction rod is used to connect the other four carriages. It is located below the connection between the carriages and drives the rear carriage to move forward steadily. With it and the semi-automatic coupler, the two carriages can be closely connected "hand in hand".

Semi-permanent traction bar source | Foshan Metro

3

Why did the subway connection break?

We learned from the official report that the cause of the accident was a rear-end collision caused by track slippage in snowy weather. So how did the subway connection break in the accident? We can understand the possible problems from the failure analysis of the coupler buffer device on Beijing Subway Line 1.

Beijing Subway Line 1 is the earliest subway line built in my country. Its subway trains use close-fitting coupler buffer devices, which include semi-automatic coupler buffer devices (Figure 1) and semi-permanent coupler buffer devices (Figure 2). Both use rubber buffers. As the service time increases, the coupler buffer devices have many problems. The main problems are: (1) The rubber sheet in the rubber buffer is permanently deformed due to aging, resulting in reduced buffering performance of the rubber buffer; (2) A large number of cracks appear in the structure, buffer box and traction rod of the coupler buffer device; (3) The buffer stop iron and cotter pin are broken and the nut is loose; (4) External factors that cause the above problems of the coupler buffer device.

Source: "Fault Analysis and Improvement of Coupler Buffer Device for Beijing Metro Line 1"

The above four reasons may lead to the intensification of the longitudinal impulse of the train, a significant increase in the coupler force, close to the yield limit of the materials used for the hook body, buffer box and traction rod, resulting in fatigue cracks in the structure, buffer box and traction rod, causing a major safety hazard of train separation. Of course, low-temperature brittle fracture of metal caused by the recent cold weather in Beijing is also possible.

In addition, according to the news released by Beijing Subway Company, "the contact rail was without electricity, some trains stopped temporarily, and some trains were delayed." Zhong Zhangdui, a professor at the State Key Laboratory of Rail Transit Control and Safety of Beijing Jiaotong University, said, "It is currently impossible to determine whether the contact rail was without electricity before the accident or whether the power was cut off after the accident to ensure safety. In addition, not every subway car is powered, and it is impossible to determine whether the power outage will cause the car connection to disconnect. Therefore, it will take time and detailed investigation to determine the specific cause of the accident."

At present, the Changping Line subway has resumed operations. The continuous snowfall for many days has caused tremendous pressure on urban traffic. I also wish the injured friends a speedy recovery.

Comprehensive source: Beijing Daily, Xi'an Metro, Foshan Metro, Guangzhou Metro, Shanghai Metro, Beijing Metro, Red Star News Reference: "Fault Analysis and Improvement of Coupler Buffer Device for Beijing Metro Line 1"