There have been a few, but they're quite rare. Two of the more prominent:

https://en.wikipedia.org/wiki/Helios_Airways_Flight_522

https://en.wikipedia.org/wiki/1999_South_Dakota_Learjet_cras...

Amazing that there are so many safeguards in place to prevent what happened to HCY 522, and yet the crew managed to miss/ignore them all. I'm sure there have been many thousands of instances where a safeguard prevented such an incident, but in this case I suppose the fates aligned.

That Helios article is just terrifying

Look at the Armstrong limit (at 62,000´), a mayor problem to overcome if the tube is going to be at the equivalent pressure of 150,000´

http://en.wikipedia.org/wiki/Armstrong_Limit

http://en.wikipedia.org/wiki/Time_of_useful_consciousness

Also look at the TUC (time of useful consciousness) that it´s below 5 secs when flying that high. That means that you have less than 5 secs to put your oxigen mask on before you lose conciousness, or behave with out knowing what is happening. Also due to the extreme low pressure, you´ll have a horrible a horrible stomach ache due to the expansion of the gases inside you. Your blood (or at least the corporal fluids exposed directly to the low pressure) will start boiling.

Not funny. Maybe one solution could be wearing some kind of pressure suit for this trips. But that is not going to be very convinient..

From the linked article:

    At or above the Armstrong limit, exposed bodily liquids 
    such as saliva, tears, and the liquids wetting the alveoli 
    within the lungs — but not vascular blood (blood within the     
    circulatory system) — will boil away without a pressure suit

As mentioned above, we have been boarding planes without pressure suits for decades, and the hyperloop has two security advantages: it has a large supply of compressed air to work with, and the tunnel can be quickly re-pressurized in the event of a severe rupture. A multitude of concurrent failures (much less likely than those in an airplane) would be necessary for a catastrophe.

> As mentioned above, we have been boarding planes without pressure suits for decades

The Armstrong Limit is at 62,000 feet. Jetliners fly at below 45,000 feet.

A dedicated hypoxia document by the FAA:

http://www.google.be/url?sa=t&rct=j&q=&esrc=s&source=web&cd=...

You got to think that a decompression will happen. Something traveling that fast inside a tube is probable to develope vibrations and material fatigue if something is not working properly (like the compressor or a air cushion), also there can be small parts released from the front pod impacting on the next one.

It doesn´t need to be a huge crash, just a crack on the hull, it will decompress that small pod in seconds.

Also what will happen when of tens of pods traveling that fast, find that there is a sudden recompression of the tube? How fast are they going to stop due to the front overpressure (the compressor is relatively low powered)?. Is not going to be a gentle stop either.

I still think that it´s a great idea, and that there will be ways of solving those problems, but it´s going to be difficult and require lots of R&D.

Obvious engineering issues for further investigation. I would just point out that fatal failure mechanisms can be pointed out in any transportation system. The acceptance criterion is a sufficiently low fatality rate.

Are you by any chance at the hyperloop team? I see that you only commented on this topic.

The great thing about the hyperloop, and other vactrain systems, is that you're at sea level... you're just not in atmosphere. The atmosphere can be restored in seconds. Sure, it'll stop the entire system and be a TOTAL pain in the arse because you'll have to restore the vacuum, but that's acceptable as long as it doesn't happen often.

Also, the Helios Airways Flight 522.