Deep under the streets of the capital, a disused railway tunnel stretches for six miles. After being shut for a decade, there are now plans to reopen the London Post Office Railway - known to many as Mail Rail - as a tourist ride.
Mail Rail was approved by an Act of Parliament a century ago, and during its heyday its driverless trains carried 12 million postal items daily on the line stretching from East End's Whitechapel to west London's Paddington. But it was mothballed a decade ago.