Technology exists to build the Hyperloop, but it won't happen anytime soon

When will the Hyperloop become an actual thing?

That's the question on everyone's mind this week after Hyperloop One successfully used an electric motor to push a sled down a test track at speeds of over 100 miles an hour. The demo lasted just seconds, in a taste of what's to come if engineers can get the whole thing built. Hyperloop One's chief executive, Rob Lloyd, estimates his company will start serving passengers in 2021, but it could be even longer than that. Here's why.

If you're not familiar with the Hyperloop, it's essentially a maglev train housed inside a near-vacuum tube. The lack of an atmosphere reduces drag and theoretically will enable passengers to zip along at airliner-like speeds. Wednesday's test didn't involve the tubes, and this time the sled was mounted on rails that won't be a part of the final product. But it did represent a first step toward a fully-functioning model Hyperloop One wants to try out before year's end.

Even from that point, though, it'll take a much larger leap to reach the point where humans get to ride in the contraption. The challenges ahead can broadly be grouped into two categories: Technological, and political. Together, these challenges mean it could be years before anyone will be criss-crossing the country in little windowless capsules.

Many of the technological problems have to do with the frailty of the human body. Acceleration, braking and banking all subject you to g-forces that only intensify the faster you go. Push beyond your limits, and you could suffer a stroke or completely black out. Actually solving these issues is simply a matter of physics, and not really all that hard. What makes them so thorny is what the solutions would imply for politics, which arguably pose the Hyperloop's biggest challenge.

You see, in order to avoid putting undue g-forces on passengers, the Hyperloop has to travel in a straight line. It can't really make turns, and when it does, the turns would have to be very gradual.

"When you're going 600 miles per hour, you can't really go around curves, and you'd have to be very flat," James Powell, a physicist who helped invent the superconducting maglev train, told Livescience in 2013.

That means in order to get anywhere efficiently, the Hyperloop might have to cut across lots of different people's land. Musk's original proposal budgeted $1 billion for acquiring permits and land rights. And, he said, the Hyperloop will require less access to land than high-speed rail, because the pylons that support the Hyperloop's elevated track are "comparable to a tree or telephone pole."

It's possible, even, that the Hyperloop could fit within the median of a major highway. Building it there would reduce the cost of having to buy private land, but it would increase the need to get something called rights-of-way - essentially, government permission to build infrastructure on public land.

This could amount to a very big, very complicated knot of policy and regulation. It's a bit like the problem facing Google Fiber, which carefully selects which cities to go into based on the costs of acquiring rights-of-way. But the Hyperloop can't pick-and-choose quite so easily: It has to go with the straightest, most direct route possible between cities.

What if some town refuses to let the Hyperloop through? Does it have to go around? If so, the train would have to slow way down to make the turn, or else the turn would have to start incredibly early - potentially requiring the Hyperloop to deviate from its intended path, calling for even more land acquisition or adjacent permits.

In this respect, the Hyperloop faces even greater constraints than high-speed rail projects. Where physics doesn't thwart the Hyperloop, NIMBY-ism might.