Exactly. Crossrail in London had some insanely difficult challenges (like threading a new subway tunnel between two existing ones with only a few cm of distance

Exactly. Crossrail in London had some insanely difficult challenges (like threading a new subway tunnel between two existing ones with only a few cm of distance between them), and hitting an enormous ancient Roman burial ground at Liverpool Street. Despite that (and many more) it is currently 6 months ahead of schedule. There's a great documentary about the project called the fifteen billion pound railway which covers a lot of these challenges.