Researchers hope they may have solved the “tunnel boom” problem as they prepare to roll out China’s latest prototype magnetic levitation train.
The newest version of the maglev train is capable of travelling at 600km/h (about 370mph). However, the train’s engineers have wrestled with the problem of the shock waves which occur as the train exits the mouth of a tunnel.
When a high-speed train enters an enclosed space such as a tunnel, air in front is compressed, like in a piston. The resulting fluctuations in air pressure coalesce at the tunnel mouth, generating low-frequency shock waves. These are colloquially known as a “tunnel boom” – a related, albeit different phenomenon to the “sonic boom” heard as aircraft pass the speed of sound. Tunnel booms pose serious challenges to operational safety, as the shock waves can disturb humans and animals nearby, as well as causing structural damage.
Now, however, researchers have discovered that placing innovative soundproofing buffers at tunnel mouths can reduce shock waves by up to 96%. This promises improvements in operational safety, noise pollution and passenger comfort, as well as safeguarding animals in the vicinity of future lines.
This was already a well documented problem for conventional high-speed trains, which travel at speeds of up to 350km/h (217mph), but it worsens significantly for trains travelling at even higher speeds because the strength of the shock wave increases rapidly and the critical length which gives rise to a tunnel boom drops off quickly. For example, a train travelling at 600km/h will lead to a boom in a tunnel just 2km (1.2 miles) long, while for conventional high-speed trains this happens only in tunnels which are 6km or longer.
The porous structure of the new 100-metre long buffers, combined with porous coatings on the tunnel body, allow the trapped air to escape before the train reaches the tunnel mouth, suppressing the boom in the same way as a silencer fitted to a firearm.
Magnetic levitation refers to the use of magnetic force to suspend a train above a guideway or rail, sometimes with a height of only 10mm, by either electromagnetic or electrodynamic suspension. The train is then propelled using other electromagnets. While conventional high-speed trains are ultimately limited in speed due to increased wear and tear of wheels against the track, the separation of track and train means that maglevs rise above such earthly concerns as friction.
Electromagnetic suspension (EMS) has the train hugging a single steel rail with a U-shaped underside. When electromagnets connected to the train – positioned in the U-shape underneath the rail – are switched on, the train is levitated by a resultant attractive force between the train and rail. With electrodynamic suspension (EDS), the train sits in a U-shaped guideway, with superconducting coils embedded in guideway and train. When the power supply is switched on, magnetic poles are induced in the coils, leading to a combination of repulsive and attractive forces which enable the train to levitate.
High-speed maglev trains made their debut in 2004 in China, running between Pudong airport and the outskirts of Shanghai at 460km/h (286mph), a speed record that still holds for rail vehicles in regular commercial service. Built using German ‘Transrapid’ technology, this service caters primarily to foreign travellers as local people prefer the much cheaper, albeit slower, metro.
However, this initial hype was soon eclipsed, as subsequent development of China’s rail network focused entirely on conventional high-speed rail. The national network is now the world’s largest in length at 48,000km (30,000 miles), with more lines under construction.
But maglev trains are now making a comeback under the state-owned manufacturer CRRC, which launched the new model in 2021. There is no mechanical noise, passengers describing the quiet hum of electromagnets and a ride smoother than a conventional train.
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Although no lines have yet been formally planned, it is widely expected that a future line will connect the capital, Beijing, with cosmopolitan Shanghai, reducing journey times from 4.5 hours to 2.5 hours, about the duration of a domestic flight between the two cities.
In China, the cost of a high-speed rail ticket is cheaper than air travel (¥600 compared with ¥1,200), unlike in many other countries. Flights emit on average seven times more CO2 than high-speed rail by distance travelled, representing a big potential carbon saving.
China is not the only place where long-distance high-speed maglevs are on the horizon. Japan also has its hopes pinned on the Chuo Shinkansen, which will link its two biggest cities of Tokyo and Osaka via Nagoya, cutting through the heart of the country. The Tokaido Shinkansen, a conventional high-speed rail line, does this journey in 2.5 hours, but it is hoped that the new maglev line travelling at 505km/h (314mph) will reduce this to just 67 minutes. It was originally scheduled to begin partial service in 2027, but inevitable delays have encumbered the project, with a new opening date uncertain.
