How Did a Kingfisher Inspire the Bullet Train? Biomimicry

The problem: booms from tunnels

Japan’s high-speed rail network, the Shinkansen, has many tunnels because much of the country is mountainous. In the early 1990s, the West Japan Railway Company (JR West) wanted trains on its Sanyo Shinkansen line to run at 300 km/h (186 mph).

There was a noise problem. A fast train entering a tunnel acts like a piston in a tube. It squeezes the air ahead of it and creates a pressure wave that travels through the tunnel at the speed of sound. When the wave reaches the far end, part of it bursts out with a loud bang known as a tunnel boom (engineers call it a micro-pressure wave). People living near tunnel exits complained, and Japan’s strict noise rules meant faster trains had to be quieter, not louder.

Making the train’s nose longer and more pointed helps, because it squeezes the air more gradually. The question was which shape worked best.

The solution: copy the kingfisher

Eiji Nakatsu, an engineer at JR West who was in charge of technical development, was also a keen birdwatcher. He knew that the common kingfisher dives from the air into water, a medium hundreds of times denser, with hardly a splash. That is a very similar problem to a train moving suddenly from open air into the confined space of a tunnel.

The kingfisher’s bill is long and narrow, and its cross-section widens smoothly from the tip to the head. When the bird dives, that shape lets water flow around it rather than piling up in front, so pressure builds gradually instead of all at once. You can see this in action in how do kingfishers catch fish?.

The engineering team tested many nose shapes with computer simulations and by firing models into tubes to measure pressure waves. The shape that performed best had a profile strikingly close to a kingfisher’s bill. The final 500 Series nose is very long and slender, with a nearly round cross-section, and it became one of the most recognizable train designs in the world.

What difference did it make?

The 500 Series entered service in March 1997 and was the first Japanese train to run in regular service at 300 km/h (186 mph). Commonly cited results of the redesign are:

These figures apply to the train as a whole, including its lighter body and other improvements, not just the nose. Still, the kingfisher-shaped front was central to solving the tunnel boom, and the story has become one of the best-known examples of biomimicry.

Other animals on the same train

According to Nakatsu’s own accounts, the kingfisher was not the only animal on board. Two other ideas were also borrowed from birds:

• Owl feathers. Owls fly almost silently because the leading edges of their wing feathers have tiny comb-like serrations that break up turbulence. The train’s pantograph, the arm that collects power from the overhead wire, was given a design with small serrations inspired by owl feathers to reduce wind noise.
• Penguin body shape. The support of the pantograph was shaped to slip through the air smoothly, an idea Nakatsu linked to the streamlined body of the Adélie penguin.

What is biomimicry?

Biomimicry, or bio-inspired design, means solving human problems by copying solutions that evolution has already tested in living things. Animals and plants have been refined over millions of years by natural selection, so their shapes and materials are often very efficient.

Birds have inspired engineers for centuries. Early aviation pioneers studied how birds shape and twist their wings, and modern aircraft winglets echo the way soaring birds spread their wingtip feathers to reduce drag. Owl wings have inspired quieter fans and wind turbines. Learn more about flight itself in how do birds fly?.

Is the kingfisher train still running?

Only nine 500 Series trains were built, because they were expensive to make. They were withdrawn from the busy Tokyo to Osaka route in 2010 and replaced by newer designs with different long noses. Shortened eight-car versions still run on the Sanyo Shinkansen between Osaka and Fukuoka, and one set has carried a pink Hello Kitty design since 2018.

The kingfisher bill lives on in the long, sculpted noses of later Japanese bullet trains, which were all designed with tunnel booms in mind.

A lesson from a small bird

The story shows how watching nature closely can pay off. A sparrow-sized bird that weighs less than 50 g (under 2 oz) helped solve a problem for a train weighing hundreds of tonnes. The kingfisher did not evolve its bill for trains, of course: it evolved to catch fish cleanly. But the physics of moving quickly from one medium into a denser one is the same, and that is exactly what made the comparison useful to engineers.