By the way, I can use my same method, (interlocking cubes + crucibles)

from the ground level.

What does that mean?

It means one can create tunnels or tracks for subways or skytrains just 10-20meter below or above the ground level.

And this is not an incremental build, an entire line unified and launched as one.

No hassles or complications. Just keep on adding metal balls as the metal melts liquifies and raise up to the volume required (overflowing from the holes) and then, cut off power to the wiring in the crucibles and you have a solidified structure however distance required. Though, probably more expensive to build...