Several companies are working to make Elon Musk's Hyperloop transportation concept a reality, including Musk's own The Boring Company. But one of these competitors may have just leaped ahead of the rest.

Arrivo has formed a partnership with the Colorado Department of Transportation to build a Hyperloop-like transportation system in Denver, with a goal of opening it by 2021. But Arrivo's system differs significantly from the Hyperloops detailed in Musk's original white paper, and previewed by multiple companies in renderings and small-scale tests.

First off, the vehicles won't travel in sealed tubes. That should make things easier in emergencies, but it also means pods won't be able to travel in a near-vacuum state and achieve the near-supersonic speeds promised by other Hyperloop developers. Instead, Arrivo envisions an open magnetized track that will parallel existing highways, and allow speeds of around 200 miles per hour.

Those aren't the only differences. Instead of sleek pods, the system will use sleds that can carry regular cars or cargo, similar to what The Boring Company has proposed for its urban tunnels. And while other developers envision Hyperloops as high-speed intercity transportation systems, Arrivo's focus is strictly local.

The Arrivo network would be more like a commuter-rail system, ferrying people from downtown Denver to outlying areas like the city's airport. The airport is 32 miles from downtown an normally takes 55 minutes to reach by car during rush hour. Arrivo wants to cut that time to nine minutes. Arrivo plans to start with a test route along the E-470 toll road, which runs past the airport. If all goes well, the company plans to start construction on a full-scale route in 2019, and begin operations two years after that.

Arrivo is the brainchild of Brogan BamBrogan, who is no stranger to Elon Musk's futuristic ideas. He was an engineer SpaceX before leaving to found Hyperloop One. BamBrogan started Arrivo after being ousted from Hyperloop One in a bitter legal fight. Hyperloop One has gone on to conduct high-profile tests, and secured an investment from Richard Branson.

Given how long it can take to complete conventional infrastructure projects like highways and rail lines, four years seems like a pretty ambitious timeline for getting largely untested transportation technology up and running. But the chance to be the first city with a functioning Hyperloop (or a close approximation, at least) is apparently worth the risk for Denver officials.