Yes, easily 3 to 10x.

> A hundred kilometer tunnel would be one of the longest tunnels ever built, and the longest train/track tunnel by quite some margin. And it would need copious safety features and auxiliary tunnels leading to the surface, even before the whole vacuum part gets added in. So it won't even completely avoid the need for buying above ground lands.

Indeed, so it's both a technical and economical non-starter. But it's not a 'dumb idea' in the sense that it is impossible. Merely impractical, too expensive and too complex and besides cheaper solutions exist (aircraft, for one, which scale much better with increasing distance than rail ever will).

One thing all of these 'dumb ideas' and the hyperloop, tidal energy and so on all have in common: they are great ways to get your grubby fingers on subsidies.