That was my leading theory, and "cross between a rail-gun and a Concorde" all but confirms it. Flight path like a Concorde, propulsion like a rail-gun. Exploit the natural vacuum above our heads without expensive tunnels.

Let's look at the attributes Elon has described publicly, and see how they compare to a launch loop:

https://www.youtube.com/watch?v=uegOUmgKB4E#t=43m13s

* could be cheaper than CA high-speed rail? Check (no right-of-way).

* "theoretically fastest way" from A-B? Check.

* can't crash? Check (w/ well-designed rails).

* immune to weather? Check.

* twice as fast as a plane? Check.

* lower energy cost than a car? Check.

* self-powering with solar panels? Check.

* stores power w/o batteries? Check.

https://www.youtube.com/watch?v=L-s_3b5fRd8#t=18m20s

* requires aerodynamic expertise? Check.

* "ground-based Concorde – as fast as a Concorde, but on the ground"? Arguably. All weight is supported by the ground, and cars take a Concorde-like path.

* rails are not needed? BZZT!! However, he may be talking about the "right-of-way" aspect of rails here.

http://www.businessweek.com/printer/articles/71590-elon-musk...

* leaves right when you arrive? Check (w/ individual pods).

https://twitter.com/elonmusk/status/224406502188916739

* isn't a vac tunnel? Check.

Of all the ideas that have been kicked around, the launch loop seems to fit best. Elon would certainly have been exposed to the idea when brainstorming for SpaceX.

My only question for the past few weeks has been, "How do you build the thing?" It doesn't inertially support itself in the air until the rotor is spun up, and the rotor can only be spun up when it's fully constructed between the endpoints. Catch-22!

There are basically three ways to do it, each with their own challenges.

Build it in the air. You would need huge numbers of lifting balloons (presumably of zero-pressure solar Montgolfier design, ala ARCA), and the portions of the system in the troposphere would be incredibly susceptible to adverse weather while under construction. You would want to have the entire thing already built, then unreel it as fast as possible. Smart tethering would reduce this risk, and the whole thing would need emergency descent parachutes for operation anyway. This seems like the least unworkable scheme, imho.

Build it in the water. Ocean currents and tides are now your problem instead of wind. The requirement to survive the corrosive salt water environment puts additional constraints on material selection. Oh, and you have to shut down all boat traffic from LA to SF. And your base station either has to be mobile (to start out in the ocean) or you have to start with a big arc out from the coastline and "tip it up" vertically. Do the math on how big the rotor endpoints would be (given reasonable assumptions about bending magnets), and you quickly realize that building them in place is the only option.

Build it over land. This seems the most unworkable – all the risk of having it fall (or worse, lose containment), and now you have to build tall towers every few thousand feet and string a high-tension support cable between them. Imagine explaining to homeowners that the "rail" we're building above their house will have parts moving at 10 km/s inside before it lifts off. Ouch.