To save someone else digging through 4 layers of Twitter/Twitpic attribution and "full size" buttons:

http://twitpic.com/cw4pqb/full

That looks like a more elaborate version of something I thought of as a kid, which was basically a vacuum tube going all around the world (one big torus if you will) with carts on super conductive magnets. I was thinking mainly of connecting Tokyo and New York and some place or other in Europe. Cart drives into the station, station gets sealed off and air pumped into it, then passengers leave and board, the doors get closed and the station sealed, then the thing speeds off at a gazillion miles per hour. Yay! Of course you'd have to start braking long before arrival, but the idea wasn't really about saving time (I could not possibly have anticipated how pampered and stupid grown ups are), it was mostly about saving energy, the idea of near zero friction just fascinated me. Though I was never sure if cooling the magnets and keeping the vacuum up wouldn't be more expensive than the energy saved.

But all that aside: bleh to passengers, think freight! If it's too complicated for people, because they won't use it if it takes as long as taking a plane... even though being a passenger on just one airplane ride is like riding a fat car for a whole year (give or take, it's nuts either way)... then automate it and use it for cargo. Especially since in that case and for a lot of products the vacuum wouldn't matter, the cargo could just ride in open carts and get picked up by robotic arms. Heck, you might even just throw it on while it's moving, and even use hooks to get cargo off without stopping the cart at all. This might seem too tricky... until you realize what marvels are going on in a bog standard HD these days, right? (though I remember a similar invention for passenger buses from MAD Magazine: something about spring-loaded seats haha)

From the image, there doesn't appear to be any vacuum. The tunnel is filled with air, which blows around the loop like a circular wind tunnel.

I don't know anything about fluid mechanics, but wouldn't there be some serious friction between the fast moving air and the tunnel wall? I'd be surprised if that resulted in less energy loss than a fast moving vehicle through static air, but perhaps someone with expertise about such things could chime in...

of course it's in comic sans

If you really want a fast transport system you need not only speed, but low latency. If you need to queue for 15 minutes while you wait for other people to get in, then it's gonna feel like boarding a plane : time wasted. What's really needed here is parallel loading of passengers into individual pods. But then you need a tube switch and multiple tubes and doors and throughways.

Neat idea but I just don't see how it's cheaper than a high speed rail project?

I agree -- I don't think that any kind of tunnel or tube approach can be done more cheaply than HSR. And I venture to predict that if it is a tunnel or tube concept, it won't succeed in linking LA and SF.

That said, if we take Musk at something close to face value, it has a major advantage that the HSR doesn't: it would be faster than the air route. That would give it at least a fighting chance to draw extremely high ridership and make up for high costs with high revenue.

And if the idea is basically sound, but the implementation is impractical on the SF-LA corridor, then it could be put into use in some other intercity connection with more favorable terrain and lower land costs.

My only guess on this is max capacity. Theoretically if the tube were large enough you could run a few of these a day and carry thousands of people on each one. If you can get the energy consumed per passenger way down, then costs may decrease and a ticket could be much less expensive.

If you could travel hundreds of miles in a half hour, you might make a lot more trips. Changing the cost structure and timing dramatically would change the way people think about long distance travel from something you do once in a while to being as common as going across town for an afternoon.

Obviously this is all far-fetched given today's technology, but I'm just letting myself daydream.

Agreed -- seems like you'd have higher construction costs, higher energy requirements, and similar right-of-way concerns.

"Closest", so we know that's not the whole story.

It looks a lot like a schematic Lofstrom (launch) loop. Just replace the rotor with air.

Is it an enclosed tube or just a special road that cars float on?

Enclosed tunnel: "air column" is a give-away.

This is essentially a vac-train, except instead of a vacuum, the air in the tube is propelled along with the pod.

There will be friction, but only between the air and the inside of the tube. I imagine there'll need to be some kind of "repeaters" to counter-act energy loss, but with so little loss, they could probably be solar-powered; which I think Elon alluded to.

Only between the air and the inside of the tube? Instead of a 100m train with air flowing around it, you get air at length of the track moving along the inside of the tube at the same speed.

I know as good as nothing of aerodynamics, but even if you get the latter flow perfectly laminar, I doubt it is a net win, as the contact area is 1000 times as large or so. But educate me.

Also, when you move cars into and out of the tunnel with the rotating air, you must take care not to 'plug' that air flow. Even if you do that only partly, there will be a pressure front (read: noise, vibrations, energy loss); each car will, for a moment get a sidewind at the speed of the train.

Here is a variant that, to me, makes more sense: it is not a vacuum, but they use the motion of the cars to push air out of the tunnel, so that air pressure drops. I don't have the vaguest idea how that would work, but if you can get it to work, it would be a nice trick.

I know next to nothing as well, but I think that perfectly laminar flow would be worst case. However if the inside of the tube had divots like a golf ball, the slower-moving turbulent vortices would push the faster-moving laminar stream away from the walls, so that the boundary layer doesn't impact the velocity of the central air column nearly as much.

Again: "I think..." I'd be happy if someone who knows more would chime in to tell me why I'm wrong. :-)

Turbulent flow would increase the overall drag in this situation. You're essentially taking energy from the free-flowing middle of the pipe and using it to drag more air along the wall. The reason why turbulence helps a golf ball is because it decreases flow separation http://en.wikipedia.org/wiki/Flow_separation. Separated flow makes a ridiculous amount of drag, and it's worth the extra skin drag to keep the flow attached.

Reply to self: we already have been building tunnels through which we move air at high speed. Looking at https://en.wikipedia.org/wiki/Supersonic_wind_tunnel#Power_r..., I read:

"The power required to run a supersonic wind tunnel is enormous, of the order of 50 MW per square meter of test section cross-sectional area."

A train would be, say, 20 square meters in cross section. That would be a GW of power to operate this. And that likely is a severe underestimate. I am sure one cannot lengthen a wind tunnel to kilometers without lots of power loss, but for now, let's ignore that.

For reference: Looking at https://en.wikipedia.org/wiki/TGV#Rolling_stock, a TGV uses at most 12MW at the relatively low speed of 300 km/h. Also, the three Gorges dam produces 22GW (http://en.wikipedia.org/wiki/List_of_largest_power_stations_...)

I think that's just the power required to accelerate the air. A wind tunnel is a once-through system, not a loop.