China set a new world record of the closing speed among two trains on April 21. This speed of 870 km/h is blisteringly fast for trains. But why is this record significant?

The record is about the speed of two "Fuxing" bullet trains running toward each other. But it's not a collision test in which the trains crash and get destroyed.

Instead, the trains run on two separate tracks side by side on parallel. So, no crashing took place in the test.

Each train ran at the speed of 435 km/h during the test. And when they passed each other, the combined speed doubled to 870 km/h. That's how the record was set.

It's widely known that high-speed railway (HSR) in China is fast. But why does it matter?

The difficult part of the test lies between the two trains. When they passed by each other at such a high speed, the air pressure between the trains became so high that they could be pushed off the rails, leading to disastrous accidents.

If you live near train tracks, you should have been warned to never come close to a fast-moving train. When the train passes, it cuts through the air in front of it, creating chaotic air flows around it, which will likely first blow you away from the train and then, unfortunately, drag you toward the train, resulting in an accident - most of the time a deadly one.

The same thing happened to the two Fuxing bullet trains when they passed by each other. As they ran faster than most trains, the air blew even harder.

If you have taken a ride on the Chinese HSR network, which usually runs at a lower speed of 350 km/h, you may have noticed the short but hard air blow.

How did the engineers prevent accidents from happening then? That's what the record-setting test was all about: getting data to study the changing pattern of air pressure when trains pass each other at a high speed.

The air flow can be hard to study. There's a specific field called "fluid mechanics" in physics that studies such a phenomenon.

"A total of nine new technologies independently developed by China have been adopted on the test trains this time," said Luo Qingzhong, vice president of Chinese Academy of Railway Sciences.