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If an airplane's passengers start yawning and chewing gum, they might not be over-tired chewing-gum executives—they might be trying to keep their ears from popping.When an airplane changes altitude, air pressure changes often make peoples' ears go pop, and yawning or chewing can help. If you saw this same thing on a train however, you might wonder what was up. Trains aren't supposed to change altitude rapidly, are they? Why would your ears go pop on a train? The answer is tunnels, and the air inside them.
如果飞机上的乘客开始打哈欠，嚼口香糖，他们也许并不是因为疲惫不堪而嚼口香糖，或许是为了防止高空耳鸣。当飞机改变飞行纬度，气压会发生改变，使乘客的耳朵发生耳鸣。然而当你在火车上遇见这样的事，你会觉得奇怪，不知道发生了什么事。火车一般不会突然改变纬度，是吗？那为什么在火车上也会发生耳鸣？答案就在隧道，以及隧道里的空气。
Of course there's air outside the tunnel too, but very different things happen when a train moves through the closed-in air of a tunnel instead of the usual, open air. Traveling across an open plain, a train simply pushes the air aside to allow its passage. In a tunnel, things aren't so easy. When a train enters a tunnel, it compresses the air in front of it like a piston. Unlike outside air, air in a tunnel can't be simply pushed aside—the tunnel walls are in the way.
当然隧道外也有空气。然而，当火车不在开阔的天地里驰骋，而是要穿过封闭的隧道时，情形则大不相同。在广阔的平原上行驶时，火车只是简单地排开空气，然后驶过。在隧道内将空气排开并是不那么容易。火车进入隧道会压缩车身前方的空气，就像活塞一样。与外面不同的是，隧道内的空气不能被推到两边——因为有隧道墙壁挡道。
Some of the air is pushed forward, all the way down to the tunnel's other end, but most of it rushes through the narrow space between the train and the tunnel walls, to fill in the area behind the train. Being forced into this narrow space makes the air rush faster, exactly as water speeds up at the base of a funnel. In fact, this tunnel air can rush backward much faster than the train's forward speed! This fast moving air creates a kind of suction on the train, lowering the air pressure inside and making your ears go pop.
部分空气从隧道一端直接被推到另一端，但大多数空气会冲过火车与隧道墙壁之间的狭小缝隙，来填补车后的空间。空气被强制压缩到狭窄的空间，会使空气运动更加剧烈，正如水在漏斗里会加速下流一样。实际上，隧道里的空气向后冲撞的速度比火车前行的速度更快。这样快速的空气流动会吸住车身，从而减小车内的气压，使乘客产生耳鸣。
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