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 this same thing happened on a train, someone might wonder what was up. Trains rarely have rapid altitude changes. Why would a person's ears pop on a train?

The answer is tunnels and the air inside them. 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.

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, filling 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.

Sources And Further Reading:

• Hamer, Mick. "Trains that Go Pop in the Dark." New Scientist. September 9, 1989, pp 63‑65.

• American Academy of Otolaryngology–Head and Neck Surgery. "Ears and Altitude." American Academy of Otolaryngology-Head and Neck Surgery. February 22, 2016. Accessed January 11, 2017.