Tunnel boom

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Tunnel boom is a phenomenon similar to a sonic boom that occurs when a high-speed train exits a tunnel. The effect occurs because a train moving at high speed compresses and displaces a great deal of air; normally this air is pushed away harmlessly in all directions. However, when the train enters the tunnel, a high pressure zone is created when the displaced air has nowhere to go. When this area of high pressure leaves the tunnel, it expands outward rapidly, creating a boom effect.

As tunnel boom can be a major disturbance (and possibly contribute to hearing loss) to residents near the mouth of the tunnel, reducing it is a major challenge for trains such as Japan's Shinkansen and the French TGV. Methods of reducing tunnel boom include making the train's profile highly aerodynamic and widening the tunnel entrance.