I'd say it was the raving of a lunatic if it wasn't Musk. He's already accomplished amazing so now he's aiming at "nearly impossible."
From an engineering perspective I have a hard time imagining how that much volume of tube is kept under enough vacuum and how to make it tamper resistant - shooting some holes in tube might be bad. And what about a big earthquake? Or a truck hitting the support beam? I don't have a problem with the PSI load on the tube (roughly one atmosphere, maximum) but it takes a LOT of vacuum pumps (and energy) to evacuate chambers that are orders of magnitude smaller than this.
You know what diffuses through almost anything when you pull high vacuum? Atmospheric hydrogen. Which is also harder to pump out. Of course if low pressure is maintained then it, along with oxygen, should not build up to flammable levels. Just noodling over possible unintended consequences.
I can see how it could be made safe, with enough braking force available in emergencies and sensors to show a stopped "car" ahead. The cars themselves need to be really well sealed and have a supply of compressed atmosphere. A leak into the vacuum tube would asphyxiate us.
I don't yet follow why it would be cheaper than a conventional train ride. Does maglev make the total work done, in a physics sense, less? Or put another way, are wheels on rails THAT inefficient by comparison?