[bluGill]

> The leak would start an implosion

Depends on the materials, but most materials have very different characteristics and won't implode. Leaks are seen in modern vacuum systems all the time, and implosions are rare. I've personally caused a leak in a vacuum system when I disconnected the vacuum hose from the pump - air rushed into the hose and no implosion!

> in particular thermal expansion of steel. The only way to deal with thermal expansion is by using expansion joins

After some thought (I've been thinking about them for a while), there are two solutions: first go far underground where the temperature is stable year round. Put the tube inside something that you do careful HVAC in to ensure there is no temperature changes. Both make the whole system even more insanely expensive than what it already is. I have zero confidence in expansion joints unless we replace them yearly which means the whole is even more insanely expensive.

Throw enough money at it and I think engineers can solve the engineering problems. However nothing changes the fact that it is insanely expensive no matter how you look at it.

The real problem isn't engineering though, the real problem is political: you can't get enough money to build the thing, and if you even try you will discover that nobody will let you build it in their backyard.