The source? Testing of the new Shinkansen ‘bullet trains’, which caused the noises by compressing air as they sped through tunnels at 150-200mph (240-320km/h). The compressed air became shockwaves, which resulted in loud booms as they reached the exit.
The problem was first solved by perforated portals at tunnel entrances, which reduced the intensity of the pressure waves in the air. The Shinkansen’s now iconic redesign – inspired by the sleek shape of a kingfisher’s beak – provided another solution, allowing pressure to build up more gradually and reducing overall noise.
The threat of tunnel booms could return, however, as trains cause stronger shockwaves by going even faster – and that is the aim of a new maglev project in China, which aims to travel at up to 600km/h (373mph). The system uses magnetic repulsion to lift trains off the track and propel them forwards, eliminating wheel-on-rail friction and enabling higher top speeds.
Thankfully, according to recent reports, a combination of techniques might nullify tunnel booms, even at those high speeds. Researchers have found that soundproofing buffers at tunnel mouths could reduce shockwaves by up to 96%, according to an article published last week in The Guardian. With a porous structure, the 100 metre-long buffers would act like the silencer on a gun, the report said, allowing air to escape before trains reach tunnel mouths.
The research was not the first to tackle the problem of maglev-related tunnel booms, which could threaten the integrity of tunnel walls, and passengers’ ear drums. Last month, a paper by a team from Central South University in China found porous ‘hoods’ on tunnels could reduce the amplitude of boom-causing micro-pressure waves by up to 86.1%. Other research has explored measures such as active suction and blowing of air from trains’ streamlined nose sections.
Solutions to the disruptive shockwaves could be good news for planners in China and Japan, where new maglev lines are being planned and discussed, adding to the seven in operation. But other challenges remain, according to experts contacted by Professional Engineering, and they will make wider deployment very difficult – especially in densely-populated countries such as the UK.
Maglev only offers significant time savings compared to high-speed rail for journeys travelling at over 250mph, said David Shirres, editor of Rail Engineer and former chair of the IMechE Railway Division’s Scottish Centre. “It’s difficult to see how the huge investment in its infrastructure and the required land take could be justified, especially as – unlike high-speed rail – it could not take advantage of the existing network to offer a variety of journey opportunities,” he said.
“I’d prefer to focus on the benefits of rail which, though an old technology, offers the most efficient transport for flows of any significance.”
There are “quite a lot” of other issues with maglev other than noise, said transport journalist Christian Wolmar. These include inflexible operation and high energy consumption, he said.
“Steel wheel on steel rail is very energy efficient, it's quite difficult to beat that,” he said. “Of course, it provides some drag, but it's not that much of a detriment.”
Maglev projects would also involve building significant new infrastructure in city centres to unlock any potential time savings, he continued – if stations were built in the outskirts instead, the lines would not save passengers much time. “At the end of the day, it's only worth it if it saves a lot of time,” Wolmar said.
During recent visits to manufacturers and operators in China and Japan, the transport commentator said new high-speed trains and metro projects were given much more focus than maglev trains under development.
“Every project that has ever been conceived has been over-budget and expensive, and technically has not worked out well,” he said. “The vehicles have to be quite light in order to be able to travel that fast and without vast energy consumption. But then, if they're quite light, they're also very vulnerable if anything gets in their way.”
He concluded: “Maybe this might happen at some point, but I can't see it becoming a wider form of technology given the enormous amount of investment required, the amount of energy required to use them, the difficulties with junctions and connections.”
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Content published by Professional Engineering does not necessarily represent the views of the Institution of Mechanical Engineers.