Crossrail has 21km of (twin) tunnels, which apparently cost 1.5B (just the tunnelling contracts). (https://en.wikipedia.org/wiki/Crossrail https://en.wikipedia.

Crossrail has 21km of (twin) tunnels, which apparently cost 1.5B (just the tunnelling contracts). (https://en.wikipedia.org/wiki/Crossrail https://en.wikipedia.org/wiki/Crossrail)