> the tunnelling machines that had all the pieces replicated inhouse

Wait... did you build your own tunnel boring machines? Or just spare parts for them?

If you have the capability to manufacture heavy machinery, building it by copying an existing design is not rocket science. They're not crazy complicated and often built or adapted to a dig specific tunnel anyways.

Now, operating a tunnel-boring machine, that's a different beast, but you'd have to do that either way. Probably should get outside help if your engineers and scientists haven't planned and dug a tunnel in their life.