Musk promises '10km Hyperloop tunnel' in 2020
Elon Musk says he wants a new 10km (six-mile) vacuum tunnel for his futuristic Hyperloop - ready for next year's speed-record competition.
Hyperloop aims to transport people in high-speed pods in underground tubes.
Mr Musk tweeted his plan after the Technical University of Munich set a new record for the fourth year running.
Its pod reached 463km/h (288mph) on the current test tube in the US, which runs for 1.6km straight. Mr Musk said the new vacuum tube would be curved.
Ultimately, Hyperloop trains will aim for a top speed of 1,220km/h (700mph).
The idea received investment from Richard Branson's Virgin group, which is developing a passenger system called Hyperloop One that it says will have a top speed of 1,080km/h (670mph).