After the cause of the rear-end collision of the Changping Line of the Beijing Metro was initially identified, the discussion in the industry around technical issues such as the signaling system began to increase.
At about 12 noon on the 15th, the Beijing Transportation Commission reported: At 18:57 on December 14, two trains collided with each other in the upward section of Xierqi to Life Science Park on the Changping Metro Line, causing some passengers to be injured.
According to the above-mentioned report, according to the preliminary investigation, the cause of the accident was the degrading of the signal of the front car due to the slippage of the track in snow, and the emergency braking and stopping, and the rear car was located in the downhill section, and the snow caused the train to slide, and failed to brake effectively, resulting in a rear-end collision with the front car.
This is more consistent with the situation that the first financial reporter saw at the accident site on the evening of the 14th that some of the carriages were squeezed and deformed, and the two carriages broke and fell off. In this accident, there was a separation of the carriage and a rear-end collision between two cars, but the relationship between the two cars is not yet known.
A compartment that has been squeezed. Photo by Ma Chenchen.
What is signal degradation?Once there is a signal degradation and the vehicle emergency braking stops, how will the vehicle scheduling arrangement on the line avoid the above-mentioned rear-end accident?In addition to the snow factor, did the rear car maintain a proper distance from the front car in this accident?
In this regard, Yicai interviewed some technical experts in related fields and tried to do some analysis from the technical level, and their analysis of the accident was different. Some experts believe that the problem may lie in the braking and the vehicle itself, and some experts believe that the accident should be a problem in the scheduling and control of the signaling system. Another expert pointed out that "internationally, there is generally a particularly bad weather, and ground vehicles need to be stopped because they exceed the capacity and conditions at the time of design."
The specific cause of this accident will be determined after a detailed investigation by the relevant authorities.
On the morning of the 15th, Yin Li, secretary of the Beijing Municipal Party Committee, stressed at the city's work deployment meeting to deal with the extremely cold weather that it is necessary to make every effort to deal with the aftermath of the accident on the Changping Line of the subway. We will make every effort to treat the injured, strictly investigate the responsibility for the accident, analyze the causes in depth, put forward targeted and specific measures, and do a good job in rectifying the problems. Further improve the emergency plan, prevent accidents, and deal with situations quickly, effectively, properly and orderly.
Signal degradation and snow slippery tracks.
According to the report, the de-escalation of the signal of the front vehicle became one of the triggers of the accident.
The signaling system of the Beijing Changping Line subway is dominated by traffic control technology. In 2022, Beijing News Broadcasting said in an exclusive interview with Gao Chunhai, chairman of Traffic Control Technology, that in December 2009, Traffic Control Technology was officially established. On December 30, 2010, the CBTC demonstration project of Beijing Metro Yizhuang Line and Changping Line were successfully opened, and the CBTC signaling system independently developed by China was successfully applied to these two subway lines, making China the fourth country in the world to have this core technology and successfully open operation.
It was only after we saw the news that we found out that something had happened to the car, and now we are still waiting for the results of the official investigation. On the afternoon of the 15th, Ms. Zhang, the manager of the ** Department of Traffic Control Technology, told the first financial network.
Ms. Zhang said that the so-called "signal degradation" is essentially an operation that follows the principle of "fail-oriented safety". If the vehicle is in a bad situation during driving, it needs to slow down or brake to avoid unpredictable results due to high-speed operation, which does not mean that the signaling system has failed and degraded. "It's like if you're driving or riding a bike in the winter, walking on the road, it's easy to slip. Rail transit is a truth, in this kind of weather, you can no longer drive at high speed, you need to keep a safe distance, brake slowly. ”
However, another signal processing expert told reporters that "signal degradation" can be popularly understood as signal failure, and the reliability of the communication link is not good. "Generally, the train will send a signal to the control center when there is a situation, which has several communication links, and if a few fail in the middle, the braking information of the accident car will not be communicated. The expert said that there are many possibilities for reliability problems, and the specific situation depends on the final investigation report of the accident.
At the fracture of the subway car of the Changping Line, the engineering personnel investigated the on-site situation. Photo by Ma Chenchen.
Snow became another important factor in the incident.
At 17 o'clock on the 14th, the Beijing Meteorological Bureau issued a yellow warning, saying that there was light snow or scattered light snow at night that day, and the minimum temperature was minus 7. In the yellow warning for icy roads and blue warnings for cold waves, there is snow and ice on the road, so pay attention to traffic safety. When the reporter rode from the exit of Xierqi subway station to the section of the incident two kilometers away in the evening of the same day, there were several tire slips. Walking on the sleepers and stones of the track in the section where the incident occurred, there was sometimes slippage under my feet.
Yicai noticed that two days before the incident, in order to cope with the low temperature and snowfall weather, Beijing Metro organized staff to overhaul and maintain vehicle equipment and elevated line facilities. According to a news release from Beijing, during the snowfall, the Beijing subway will open 252 sets of turnout snow melting devices to prevent the turnouts from icing;The ground and elevated lines will be operated in manual driving mode, and the speed of trains will be reduced in a timely manner according to the snowfall conditions, and technical backbones will be arranged to take additional trains to inspect the lines to ensure driving safety.
Zhao Jian, a professor at the School of Economics and Management of Beijing Jiaotong University, told the first financial reporter that the puzzling thing about the accident is that, theoretically speaking, the front car has been degraded and stopped by emergency braking, and when it does not leave the braking zone, the rear car should not be able to enter the interval. Even if there are objective factors such as downhill sections and slippery roads in snowy days, they should be carefully evaluated and considered in the early stage, and effective measures should be taken as far as possible to avoid the superposition of multiple unfavorable factors, which may lead to accidents.
Will the signaling system of the Changping Line of the Beijing Metro reserve sufficient braking distance and time according to special conditions such as extreme weather and downhill?
Ms. Zhang said that in reality, the operation of the signal system will be affected by a variety of factors, and it is difficult to generalize. For example, if you set the train interval between 90 seconds and 180 seconds, the braking distance will be different. If the interval is set to 6 minutes, the braking distance can be longer. The signaling system designs a relatively reasonable braking distance and speed based on the established intervals, and this is largely dependent on how much capacity the setter needs to transport the crowd out. In addition, factors such as the cooling and shrinkage of the steel structure in rainy and snowy weather can also affect the slip coefficient of the vehicle and track, and these complex factors will also interfere with the final result.
Zhao Yixin, dean of the Urban Transportation Research Branch of the China Academy of Urban Planning and Design, and a professor-level senior engineer, said in an interview with Yicai that subway rear-end collisions are not common. Since the train operation mainly relies on the signal control system to dispatch and command, such as extreme weather such as heavy snow, the signal control system should effectively maintain the safety requirements to ensure the safety of the train operation. In China, because rail transit is mostly underground, the above-ground track mileage in the northern region is relatively short, so it is even more rare for subway rear-end accidents to occur in extreme weather.
In response to the rear-end collision accident between two trains on the Changping Line of the subway, Zhao Yixin believes that the operating speed should have taken into account the usual rain and snow weather, and it is safe under normal circumstances. However, in extreme cases, due to the snow on the tracks, the braking distance of the manual brake is extended, and the emergency braking time is delayed, which may lead to the shortening of the distance between the two moving trains, and even the occurrence of rear-end collisions.
In Zhao Yixin's view, the accident sounded the alarm for cities in the north that are not underground and have closed operating tracks. First of all, because subway rear-end collisions are extremely rare, there are often gaps in the plans for similar safety risks on smart tracks, including digital monitoring. More importantly, the subway dispatching and command center should take corresponding emergency plans for extreme weather, and can consider improving the safety level, increasing the distance between trains, reducing capacity, and strengthening the safety and reliability of communications and related equipment and components.
How do you balance operational safety with capacity demand?
After the accident, many netizens expressed concerns about the safety of the subway in rainy and snowy weather. In this regard, Zhao Jian said that subway accidents are still extremely unlikely events on the whole, but road traffic is prone to potential safety hazards under the same weather conditions, and people have become accustomed to it. He believes that in this accident, the simplest solution is to increase the interval between vehicles, which may largely avoid the occurrence of dangerous situations.
The first financial reporter inquired about the official ** of the Beijing Metro, and the train timetable of Xierqi Station of the Changping Line showed that the train interval ranged from 2 minutes to 10 minutes, such as 17:49 and 51, 18:54 and 56, 20:49 and 51 on weekdays to Changping Xishankou, and the interval between the two adjacent subways was shorter, only 2 minutes.
A technical person in the field of rail transit in Shanghai told reporters that the information system of rail transit should add more scenes, including scenes in extremely bad weather, and the information system should set different braking parameters for different scenarios, increase the separation distance between the front car and the rear car to more than 1 km, and appropriately relax the departure time, which will be improved.
In the future, in the face of train operation under similar extremely bad conditions, manual intervention can be adopted, in addition to increasing the time interval of train operation, passenger waiting and safety guarantee should also set up corresponding plans. Drawing on international experience, it is possible to set up heating devices for the tracks, use snow melting agents, etc. to clean the tracks and help improve the adhesion coefficient of the tracks under extremely harsh climate scenarios. The technical source said.
According to Article 32 of the Notice of the Ministry of Transport on Printing and Distributing the Measures for the Organization and Management of Urban Rail Transit (Jiao Yun Gui 2019 No. 14), in case of bad weather, relevant personnel can take measures such as strengthening lookout, speed limiting, suspension, and station closure in a timely manner according to the situation, and shall organize driving adjustments in accordance with the following requirements:
For surface and elevated lines, the speed of the train should not exceed 60 km h when the wind force reaches level 7 in the wind affected section, the train speed should not exceed 25 km h when the wind force reaches level 8, and the train speed should be suspended when the wind force reaches level 9 and above.
In case of fog, haze, rain, snow, sand and other bad weather when it is difficult to look, the ground and elevated line trains should turn on the headlights, run at a limited speed, and sound the horn in a timely manner. When the lookout distance is less than 100 meters, 50 meters and 30 meters, the running speed of the train should not exceed 50 km h, 30 km h and 15 km h respectivelyWhen the lookout distance is less than 5 meters, the driver should stop immediately. When the driver cannot see the signal display and the position of the turnout, he should stop for confirmation, and it is strictly forbidden to speculate on driving.
The picture shows the Xierqi subway platform on the 14th, and passengers were evacuated from the subway tracks in the snow. Photo by Ma Chenchen.
After the rear-end accident, the Beijing Metro said on the morning of the 15th that in order to ensure the safety of ground and elevated line train operation in extreme weather, all ground and elevated lines adopt manual driving mode, take speed reduction operation measures, and the departure interval will be appropriately widened, and the bus along the line will increase the capacity of the connection, and passengers can choose to travel by bus on demand.
Appropriately widening the departure interval helps to ensure the safety of vehicle operation, but the ground transportation is inconvenient in rainy and snowy weather, and the people's subway travel demand will be more, which forms a contradiction.
Mr. Zhang, a party who experienced the accident subway danger, told reporters that he usually takes the Changping Line from Xitucheng to Zhuxinzhuang Station to commute, and usually starts at 5 o'clock in the evening, and the subway enters the rush hour. If it rains or snows, there will be more people on the ride. In particular, there are many Internet companies around Xierqi Station, and there is always a long queue at the door of the subway car as soon as you arrive at the station. "In rainy and snowy weather, you often have to queue up hundreds of numbers to call online car-hailing, and you may not be able to get on the car if you wait for an hour. It's better to take the subway, there are many trains, the time is guaranteed, and the station is not cold. ”
Zhao Jian believes that there is a contradiction between the rainy and snowy weather that puts forward longer requirements for subway clearances, and the real needs of people who travel require more frequent subways in such weather. To solve this problem, we must return to the problem of urban and rail transit planning. "With a population of about 40 million, the Tokyo metropolitan area is smaller than Beijing, and the rail transit mileage in 2017 was 2,700 kilometers. Beijing's current urban rail mileage is less than 1,000 kilometers, far from the demand for high-quality rail travel. ”
The "Major Infrastructure Development Plan for the 14th Five-Year Plan Period of Beijing" released in 2022 proposes that by 2025, the total mileage of urban rail transit operation will strive to reach 1,000 kilometers, and the proportion of rail transit trips in the central urban area will reach 187% or more. In addition to expanding the scale of rail transit, Zhao Jian also believes that all localities should closely combine urban planning and rail transit construction, so that more people can choose this efficient and intensive travel mode and reduce the pressure of urban road traffic.
Potential problems in the industry.
According to public information, subway rear-end collisions also occur occasionally in other cities around the world.
On June 22, 2009, two subway trains in Washington, D.C., collided in the northeast of Washington, during rush hour, and the accident caused the rear car of the front train to be severely deformed, and the damaged car of the subsequent car climbed on top of the previous train, causing the worst accident in the operation of the Washington Metro system in more than 30 years.
Investigators in the Washington subway crash found that signal failures in computer systems and human error may have been the two main causes of the subway accident.
Zheng Gang, an associate researcher at the Integrated Information System Technology Laboratory of the Institute of Software of the Chinese Academy of Sciences, once wrote about the rear-end collision of the Washington train, saying that since the beginning of the 21st century, China's urban rail transit has also entered a period of great development. With the continuous improvement of subway train transportation capacity and transportation efficiency, the continuous increase of transportation density, and the acceleration of train speed, the continuous improvement of the degree of automation of train operation control is required, and the safety and stability of the subway train automatic control system is particularly important.
Zheng Gang believes that on the one hand, people are the first element to ensure driving safety. The late-mover advantage of China's urban rail transit determines that a large number of advanced automated equipment (such as ATC system) are enabled, and its train overspeed protection (ATP), automatic driving (ATO) and other functions are used to ensure driving safety, but the role of people is often ignored.
On the other hand, from the technical level, the current domestic subway train control system products from different manufacturers, in the signal transmission form and standard, speed control mode, communication network and other aspects are different, even in the same city of the subway vehicle system is not the same, so it is necessary to actively promote the standardization of related technical equipment safety;Strengthen the testing, testing and certification of products.
In addition, due to the fact that some subway trains in China have been operating for many years, the operation control system of the train needs to be upgraded to improve the automatic control level of the speed operation of the subway train and the real-time monitoring, diagnosis and control ability of the train to the operation status from many aspects such as signal transmission, operation control, command and dispatch, communication network, etc. Zheng Gang suggested that the upgrading and transformation of the old subway train control system should be accelerated.
Last year, an article published in the "Rail Transit" by researchers such as Beijing Metro Operation *** said that the traditional dispatching and command mode divided by the line as the command unit has been unable to meet the development needs of network operation, combined with the current conditions of the Beijing line command center, it is recommended to optimize the development and construction plan of the dispatching and command mode.
According to the analysis of this paper, with the improvement of the scale of the line network, the current line operation gradually presents different characteristics from the single-line operation, which has four characteristics: first, the enhanced correlation between lines leads to a wider impact of emergency events;Second, the analysis and development law of passenger flow changes in time and space caused by the increase of transfer nodes and the increase in passenger flow exchange volume are becoming more and more complexThird, as the core transportation carrier of the city, whether it can ensure the normal operation of the subway and emergency response will have a more far-reaching impact on urban safetyFourth, the transportation of multiple operating entities increases the difficulty of coordination of operation management.