Trains blasting through tunnels at high speed can sometimes end in a loud “tunnel boom” — but it’s not a sonic boom in the jet-sense. Here’s what causes it, why it matters, and how engineers are solving it. What causes the boom: • As a fast train enters a tunnel, it compresses air in front of it — like a piston pushing air.  • That creates a pressure wave that travels down the tunnel.  • When that wave reaches the tunnel exit, the sudden release of compressed air causes a shock wave and sound — loud, low frequency, and sometimes startling.  Why it matters:  – Noise pollution: people living near tunnels can be disturbed.   – Structural & comfort issues: pressure changes affect train integrity, passenger ears, tunnel portals.  How engineers are solving it: • Adding porous buffer structures or coatings at tunnel entrances so air can escape gradually rather than all at once. Up to ~96% reduction in peak pressure has been reported.  • Tunnel “hoods” / portal extensions that smooth ...