Academic Reading — Test 129

Reading passage

For more than a century, the railway has been bound by a single stubborn limitation: the steel wheel pressing against the steel rail. This contact is what allows a conventional train to grip the track and move forward, yet it is also the source of friction, vibration and wear. As speeds rise, the energy lost to this resistance grows steeply, and the mechanical parts that transmit power to the wheels suffer ever greater strain. Magnetic levitation, commonly shortened to maglev, was conceived as a way to escape this constraint entirely. By lifting the vehicle clear of the track and driving it forward without any rolling contact, engineers hoped to build a transport system that was quieter, smoother and capable of far higher speeds than traditional rail. The principle that makes this possible is the behaviour of magnets, which attract or repel one another depending on the orientation of their poles. A maglev train carries powerful magnets, and the guideway along which it travels contains coils or magnetic materials that interact with them. When the system is energised, the forces between vehicle and guideway lift the train a small distance above the surface, typically only a few centimetres. Because the train no longer touches the guideway, there is no rolling friction at all. The only meaningful resistance that remains is the drag of the air through which the train passes, and this becomes the dominant factor limiting top speed once levitation has been achieved. Two broad approaches to levitation have been developed. The first, known as electromagnetic suspension, uses conventional electromagnets fitted to the underside of the train that reach around the edges of the guideway and are pulled upwards towards it. This attractive force is inherently unstable, so the gap between train and track must be measured and corrected thousands of times each second by an automatic control system. The second approach, electrodynamic suspension, relies on superconducting magnets aboard the train. As the vehicle moves, these magnets induce currents in coils set into the guideway, and the resulting repulsion pushes the train upward. This second method is naturally stable but produces no lift while the train is stationary, so small wheels are needed to support the vehicle until it gathers enough speed to rise. Levitation alone, however, does not move the train forward. Propulsion is achieved by what is, in effect, an electric motor unrolled and laid flat along the route. In a familiar rotary motor, a magnetic field spins inside a fixed casing and turns a shaft. In a maglev, that same arrangement is stretched out so that the moving part is the train itself. Electrified coils built into the guideway generate a magnetic field that travels along the track like a wave, and the magnets on the train are drawn along behind it, chasing the field but never quite catching up. By controlling the speed at which this wave moves, operators can accelerate, cruise or brake the vehicle with great precision. Crucially, the energy that drives the train is supplied to the guideway rather than carried on board, which removes the need for heavy engines within the carriages themselves. This design carries several consequences. Because no part of the propulsion system rubs against another, the components wear out far more slowly than the wheels, axles and gears of an ordinary train, and maintenance costs over the long term can be correspondingly lower. The absence of mechanical contact also makes the ride remarkably smooth and quiet, and it allows the train to climb steeper gradients than a wheeled vehicle, which would otherwise lose grip. Braking can be accomplished electrically by reversing the travelling field, so that the motor itself slows the train and much of the energy of motion can be recovered rather than wasted as heat. These advantages explain why maglev has long been regarded as a glimpse of the railway's future. Yet the technology has not spread as widely as its admirers once predicted. Building a maglev line is expensive, for the guideway must be manufactured to fine tolerances and embedded with electrical equipment along its entire length. A maglev track cannot share the existing network of conventional rails, so an entirely new route must be constructed wherever the system is introduced. The superconducting magnets used in some designs must be kept extremely cold, which adds further complexity and cost. For these reasons, maglev has so far been adopted only on a handful of short, high-profile lines, chiefly in Asia, where the promise of very high speed has been judged worth the considerable investment. Whether it will eventually displace the steel wheel on a larger scale remains, for now, an open question.