Construction work has begun on the biggest transport project since the Channel Tunnel, London's £16bn Crossrail.
Two 25 mile tunnels are to be bored under the city, linking east to west and connecting Heathrow airport, the West End, and Canary Wharf. The first trains are expected to run in 2017.
But the project has faced some criticism given the pressure on public finances.
Tom Symonds reports.
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