How was it tunnelled, most tunnelling machine are large, large enough for things to have enough space to work.

The conveyor belt to remove the debris, the machinery to place the concrete segments to form the wall.

I guess it could be shrunk, but there wouldn’t be space for people to get through

Mention is made of the North London tunnel, but in preparing for the 2012 Stratford Olympics pylons there were replaced by an Underground tunnel too, and there was a lead time of 7 years from the Stratford winning.