Friction--it's a real bother, especially for trains. Even as we make our rail faster, it's still tethered by the fact that their wheels must grind along a track. This is the problem that maglev trains are supposed to solve, but maglev trains are expensive and haven't caught on. But a new prototype from Japan would eliminate friction, and all the pesky technological requirements of a maglev: It would simply fly very close to the ground.
It's being called the Aero Train  and it's prototype was developed as a proof of concept for a paper  given at the IEEE International Conference on Robotics and Automation. It works on the principles of the "wing-in-ground effect ," which is that as planes get very low, drag decreases significantly. Take one of these planes and attach some passenger cars to it--voila!--you have yourself a very efficient flying train that doesn't require that much energy to move. Here's a video of the prototype in action, flying just inches above the ground:
It requires a pilot--or a computer--to control roll, pitch, and yaw, a far more advanced skill set than normal train driving. And, of course, it's many years away from any sort of completion. But it's efficient enough that it could be powered with just wind turbines or solar panels, while still zipping along.
When you think about our technological future, don't forget we probably have no idea how things will work in 20 years. They probably won't be what we expect. On the other hand, decades ago the Soviets developed a ground-effects plane called the Ekranoplan  that was supposed to "eliminate cargo ships," and we still have a lot of cargo ships plying our waters.
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