There was an interesting documentary on the BBC a year or so ago following the CrossRail project. One segment in particular was fascinating - a manoeuvre they called &quot;threading the eye of the needle&quot;. The Tunnel Boring Machine (TBM) had to go through an gap between an existing underground station at the bottom, and a tunnel for the down escalator at the top ... tolerances were razor thin, much less than 1m either end.<p>Image that shows what they did here: <a href="http:&#x2F;&#x2F;s3files.core77.com&#x2F;blog&#x2F;images&#x2F;2015&#x2F;01&#x2F;Crossrail_Planning_Jan15_3.jpg" rel="nofollow">http:&#x2F;&#x2F;s3files.core77.com&#x2F;blog&#x2F;images&#x2F;2015&#x2F;01&#x2F;Crossrail_Plan...</a>