High-speed, near-supersonic transport system.
The Hyperloop (or something similar) is, in my opinion, the right solution for the specific case of high traffic city pairs that are less than about 1500 km or 900 miles apart. Around that inflection point, I suspect that supersonic air travel ends up being faster and cheaper.
Tesla, SpaceX, SolarCity, PayPal…
More on Musk’s Blog.