The rear-end collision caused the Changping subway to be split in two, why did the carriage break, and where did the injury come from?

1. Introduction

Last night, the last two sections of the Beijing Changping Line train (subway) were separated from the front car, and the cause of the preliminary investigation was the rear-end collision of the train, which caused 515 people to be sent to the hospital for examination and 102 people were fractured, but fortunately no one died, which is also a great luck in misfortune. At present, 423 people have been discharged from the hospital, 25 people are under observation, and 67 people are hospitalized.

Preliminary findings: Rear-end collision

Some students feel that the train is running very fast on the track, and it can be found from the previous accidents that once the problem is more serious, why is the accident relatively light this time? In addition, there have been rear-end collisions, why have we not seen the carriage being seriously squeezed and deformed? Since the carriage has basically not been deformed, and the people in the carriage have not been squeezed by the carriage, why are so many people injured?

The subway is broken in two

2. Accident restoration

Based on the information reported by various media, the current known information is that the location of the train break is the separation of the last two carriages from the front car, and there is also a large impact force when the accident occurs. The impact force here is not caused by the impact of the object, but by inertia.

The last two carriages detached

According to other media reports, the train had just started at the time of the accident. Of course, this just-initiated judgment has a very subjective color. No matter how much it was, it is certain that the train is still accelerating at this time, and the speed is not fast enough.

The accident occurred shortly after the vehicle was started

Depending on where the accident occurred, the train had already left the station for 2km and passed another 3.4km before reaching the next station. So, it can be seen that the distance between the two stations is very short. After the train left the station, for some unknown reason, it stopped for a moment. Then the statement in the picture above appeared: not long after the start, a loud bang was heard, that is, a rear-end collision.

3. The connection of the carriage

The fracture of the coupler is the more confusing part of this accident. There is a train connection structure that has been used for more than 100 years, called Janney's coupler, which was invented by the American Janney, and some students may mistake it for Zhan Tianyou from the name. This kind of Jens coupler is mainly used in ordinary trains, like the old green cars, wagons and the like. Although it is stable and reliable, it is not suitable for high-speed, closed and other conditions.

Jen's coupler

Therefore, the carriage connection of high-speed rail and subway is not a Jens coupler. From the inside, the subway car connection is a relatively spacious aisle, and the two pedals under the feet can move relatively, in fact, the entire interface (carriage connection section) should be able to have a certain relative movement. Rigid connections are more likely to cause damage to the joints.

Subway car junctions

The subway connection consists of a coupler and a buffer, which is mounted on the underframe traction beam, which is under the foot pedal in the picture above. There are three types of couplers: automatic, semi-automatic, and semi-permanent towbar. Unlike the Jens coupler, the metro coupler has a tight fit, similar to a rigid connection. If it is obviously not suitable to make a direct connection like this, it is equipped with a buffer, so that when the coupler is subjected to a constant change of force, the buffer can play a good role in cushioning and protecting the overall structure.

Coupler structure

Normally, the coupler is in a tensile state, but from a structural point of view, a crushed deformed tube is specially designed for it. If you are under too much pressure, this part will simply break. This should be to prevent the train from rear-ending, and try to protect the carriages in front and behind the car away from the point of impact.

4. Accidents caused by rear-end collisions

As mentioned earlier, the broken train stopped for a while, and a loud bang was heard shortly after it started again. Judging from the structure of the coupler, it is unlikely that it will make a loud noise. Some students may have said that during the tensile test of material mechanics, the sound is still quite loud, and you will be startled if you don't pay attention. The mechanical test of materials is ideal, and all aspects are well controlled, so the energy loss is relatively small. If the subway coupler is broken, it must be a perennial accumulation, and the internal damage slowly accumulates. In addition, the coupler is under the action of fatigue load, and the force is not simply the direction of tension, but there is bending and torsion. At this time, if a certain position of the coupler is broken (generally the connection is a weak link), the sound will not be "loud". The sound of bending and breaking something is much smaller than stretching.

Tensile test of material force

Some people on the Internet have long suspected that the accident was caused by the rear car rear-ending the front car: after the front car stopped for some reason, the rear car had already set off, and the time interval between the two cars was 2 minutes. Not long after the car in front started, it was rear-ended by the car behind. It was also confirmed later.

A loud noise is the sound of the train rear-end, and only this sound can spread far and be called a loud noise. In addition, the media reported that the last two cars fell off from the front car because the energy was quickly transmitted forward after being hit by the rear car, but due to inertia, the structure near the impact point would be unbearable first. In the coupler structure just introduced, the connecting coupler of the penultimate and third compartments begins to work and fails, resulting in the disconnection of the entire coupler. The front carriage has been activated and continues to move forward for a period of time due to inertia, while the rear carriage lacks connection and active power, and the distance forward is shorter.

Preliminary results: rear-end collision

There were no fatalities in the accident, a big reason is the slow speed of the subway, this section is only more than 5 kilometers, assuming that the time interval between the two stations is 5 minutes, then the speed is about 60 km/h. Assuming that the rear car realizes that the situation is not right, it adopts emergency braking and the car in front has started to start, so the relative speed of the two cars during the rear-end collision is lower, which should be less than 10km/h.

5. Where does the injury come from?

Although the relative speed of the two cars is very small, the overall mass of the train is very large, so the movement of the impact is still very large, and the loud noise is a proof. After the car in front is hit, the people in the car fall backwards due to inertia, especially the people in the last two carriages, and the impact is the greatest. It's as if you're standing still, but the car under your feet is moving forward, and the friction on the soles of your feet makes you fall. In this process, especially the people at the back of the carriage will be directly crushed by the bottom of the carriage, and the people in front will also be hit by the seats, armrests, etc. during the fall, resulting in fractures. It's like a bus braking suddenly.

The bus brakes suddenly

It is also dangerous if the location of the station is located at the junction of the carriages. The relative displacement of the junction is greater and less likely to stand firm. This fall is about the same as falling from a height of about 1 meter without preparation.

6. Summary

The preliminary cause of the accident has been ascertained, and in fact it was not the coupler problem, but the rear-end collision of the train. Although the speed of the two cars is slower when they collide, the total kinetic energy is still large relative to the human body, considering the mass of the train. The people in the carriage fell due to inertia and stumbled, resulting in many fractures.