When the train moves forward it is replacing space which is taken up by air with space taken up by train. The air in the way of the train needs to go somewhere, and usually there is plenty of space around the train for it to go. But when the train is in a tunnel there isn’t anywhere for the air to easily go so it tends to get compressed, raising air pressure as the train rams its way down the tunnel.
This is why the air pressure on your ears increases in the tunnel and drops as you get near the end where the air is escaping. Sometimes train tunnels are designed with areas for air to escape to reduce this effect.
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