[9nGQluzmnq3M]

This proposal would likely be a lot more economic if they could reuse existing holes, like abandoned mines. However, mines tend not to dig down vertically and the cost of retrofitting and maintenance would be non-trivial.

[dx034]

The vertical train approach of terrament (linked above) could theoretically work with that. But we'll probably have to wait for the first prototype to see if it can really work with existing infrastructure.