Crossrail: A tour through the tunnels beneath London
It is exactly four years until the first section of London's new Crossrail line is due to open.
The £14.8bn project is set to boost the capital's rail capacity by 10%, once it becomes fully operational the following year.
Up to 24 trains will run every hour, each carrying up to 1,500 passengers during peak hours in the central section between Paddington and Whitechapel.
The tunnel-boring machine will break through at the new Liverpool Street Station some time after Christmas.
As the construction site started winding down for the festive break, Transport Correspondent Richard Westcott was given access to the new tunnels under the city to see how work was getting on.