Here’s a thing I became mildly obsessed with this weekend (thanks to the new issue of Smithsonian): “tunnel boom.” Tunnel boom is a common phenomenon in high-speed rail — like, real high-speed rail a la Europe and Japan, not the Acela Express — when a train entering a tunnel at very high speed compresses air ahead of it at high-pressure. The high-pressure wave is pushed out the other end of the tunnel, where it expands rapidly in the lower pressure environment. It’s basically like thunder, where air is heated rapidly (thus expanding rapidly) thanks to lightning.
In Japan, this problem was “cured” by the development of a new train shape based on birds’ beaks. Instead of air being compressed in front of the train, the new design parts it to each side. And suddenly the Japanese countryside is a much quieter place. Japanese tunnels tend to be narrower than Europe’s, making the boom more extreme, but tunnel boom still exists in Europe along the route of the ICE 3, the 200 km/hour high-speed train traversing between Netherlands, Germany, Belgium, and France (above).
Somehow, after an afternoon of searching, Japan doesn’t seem to have the same level of railfanboyism, so, for now, we’ll just have to make do with the rumbles of the still pretty cool ICE 3.