London's Crossrail built with 'Welsh expertise'
What is 35 metres underground, will be used by 200 million passengers a year and costs almost £15bn?
The answer is London's newest railway system, Crossrail or the Elizabeth Line as it will be known.
The 200 metre (650 ft) long trains will each carry up to 1,500 passengers on an east-west route taking in more than 60 miles (96 km) of track.
But why should anyone in Wales care about more investment in London transport?
That is a question I put to the project chairman Terry Morgan, originally from Cwmbran, and Welsh Secretary Stephen Crabb as I joined them on a tour of the new Paddington station.