A plan to build an ultrafast Hyperloop One tube train has been given "verbal [government] approval" to connect large cities on the East Coast, tech entrepreneur Elon Musk says. He adds that the system would whisk passengers from New York to Washington, D.C., in 29 minutes.

After his tweet about the plan set off intense interest, Musk added a clarification, stating, "Still a lot of work needed to receive formal approval, but am optimistic that will occur rapidly."

We're seeking more details from Musk and his companies that are involved in the Hyperloop project — from the vehicle unit to the Boring Co., which would dig the tunnels. He did not specify, for instance, which agency had given its approval or when construction might begin.

Engineers who are working on the project have the goal of sending pods through a tube at speeds of more than 700 mph, using magnetic levitation and an electric motor in a vacuum environment.

"With Hyperloop One, passengers and cargo are loaded into a pod and accelerate gradually via electric propulsion through a low-pressure tube," the company says. "The pod quickly lifts above the track using magnetic levitation and glides at airline speeds for long distances due to ultra-low aerodynamic drag."

Musk said that Hyperloop's planned underground route would run from New York to Philadelphia and on to Baltimore and Washington — the same heavily used route that is serviced by Amtrak trains. And he added that, as with traditional train stations, the Hyperloop would travel from city center to city center.

The stations, Musk added, would have "up to a dozen or more entry/exit elevators in each city."

The Hyperloop underwent its first public test last summer, with a successful run at its 500-meter test track in the Nevada desert. It cleared another hurdle last week, when it completed the first full systems test in a vacuum environment.

"The vehicle coasted above the first portion of the track for 5.3 seconds using magnetic levitation and reached nearly 2Gs of acceleration, while achieving the Phase 1 target speed of 70mph," Hyperloop One said in a news release. "The company is now entering the next campaign of testing, which will target speeds of 250 mph."

Musk made his statements about the new proposal in the same week that he has been in Washington to attend the International Space Station Research & Development Conference, where he was a featured speaker.

The Northeast Corridor route Musk outlined Thursday wasn't included in a list of 11 routes Hyperloop One published in the spring. That list mentioned other areas with heavy traffic, including Los Angeles-San Diego, Miami-Orlando in Florida and Seattle-Portland, Ore.

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