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 c

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 "threading the eye of the needle". 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. Image that shows what they did here: http://s3files.core77.com/blog/images/2015/01/Crossrail_Planning_Jan15_3.jpg http://s3files.core77.com/blog/images/2015/01/Crossrail_Plan...