system
Given the following post, respond as if you're commenting on Reddit:
human
Why do our ears pop when a train we’re riding enters a tunnel? Shouldn’t the pressure inside be equal to the pressure outside side since it’s open to the air?
gpt
A train in a tunnel drives a pressure wave in front of itself with a lower-pressure region behind.  It's pushing and dragging air through the tunnel and the air can't flow freely and quickly enough past it to completely equalize the pressure.  With a combination of too long a tunnel and too tight a fit between train and tunnel, that pressure wave ahead of the train will actually strengthen into a shock wave.