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While arguing against my friend today about the turbulence appearing between train's wheels. I told that the train pushes air aside during its motion through the tracks. But, he rejected it. Then, what's happening in air (other than air drag) when a fast moving heavy object slices through it?

How does the motion of air look like in there?

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It cuts the air if it's a bullet, or sometimes drags all the air around the object forward if it's a train. It's hard to say theoretically, do you live by a train track? Just blow some smoke on the track and watch when a train goes by. – Ron Maimon Sep 18 '12 at 13:02
up vote 2 down vote accepted

Remember that air is made up from air molecules (well, oxygen and nitrogen molecules) and the interaction of the train with the air is ultimately down to collisions between air molecules and the surface of the train. When the train is moving the air molecules at its front bounce off faster than they hit, and these faster moving air molecules in turn collide with other molecules and accelerate them as well. The end result is that the front train is surrounded by a blanket of faster moving air molecules and accordingly the pressure of the air in front of it rises. Air flows away from this high pressure zone, which causes the wind as the train passes.

When viewed in this way I'd say that "the train pushes the air out of the way" is a pretty good description.

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To add to John Rennie's answer. When the train goes past there is also a train sized hole in the air which the air molecules will move into, so there is an element of "pull" as well - or at least a low pressure area created alongside and behind the train.

With high speed trains this is a serious effect - there are safety films of test dummies placed alongside a train line being pulled off their feet and under a 150mph/250kmh train.

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