The shape can also make journeys more comfortable for passengers. Think of a recorder, rather than a trumpet. The other issue is that trains run in both directions, and the best shape at the entry to a tunnel would actually exacerbate the problem at the other end – where a series of holes are required to help dissipate micro-pressure waves. But in Japan, where high-speed trains pass through mountainous areas, there often isn’t enough space between tunnels for this approach to work, and slowing down so often would have a detrimental effect on journey times. “Imagine a trumpet-like flare at the front of the tunnel,” says Alan Vardy, of the University of Dundee. Otherwise, track designers can build ‘entry portals’ to their tunnels. The simplest solution is to simply slow down a little when approaching a tunnel. Read more: The strange tale of the hovertrain, the British hyperloop of the 1970sĬhanging the shape of the trains is not the only way to combat this problem. The ideal nose shape would actually be a half-sphere, but practical constraints – like having somewhere for the driver to sit – make this difficult. The ALFA-X design seeks to minimise this by reducing the cross-sectional area at the front of the train, meaning it takes longer for the pressure wave to build up to begin with. The flatter the front of the train, the quicker a pressure wave builds up in the tunnel, creating a bigger change in pressure when it exits. “The reason for the 'odd' nose shapes is to decrease the gradient of a pressure wave which is created as a train enters into a tunnel, with a view to decreasing micro-pressure wave emission at the tunnel exits,” says Soper. In the 1970s, Japan's Shinkansen created problems with cracked windows on nearby houses, and sonic booms that could be heard up to 400 metres away. As trains leave a tunnel, the sudden pressure drop can create micro-pressure waves.
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