19 August 2015 Last updated at 08:03 BST
The bosses of Crossrail, linking trains from Essex to Reading underneath the capital, said construction would create 55,000 jobs across the country.
Bob Thompson, of engineering company Keller Group, said it had to recruit more young engineers to carry out the work.
Martin Buck, of Crossrail said the project was "providing an awful lot of opportunities for contractors that maybe are not so used to working in London".
A Coventry firm is one of many benefitting from an underground railway scheme in London.