Subselene: Lunar nuclear powered melt-tunneling for high-speed transit tunnels
osti.gov
osti.gov
Tunneling with nuclear machines on Earth had been under consideration for decades before [1]. It's not entirely clear why it was abandoned, but most likely something didn't work out in practice like it was supposed to do in theory.
[1] https://nationalinterest.org/blog/buzz/americas-mad-scientis...