Engineer Rob Bell unearths the history of the Central Line, learning that Bank's platforms are curved to avoid the Bank of England's gold vaults. He investigates the most tragic night in the Tube's history: a disaster during the Second World War in which 173 people were killed at Bethnal Green station while seeking shelter from an air raid. At 46 miles, the Central Line is the longest of all tube lines, connecting east with west and transporting 260 million passengers a year, making it the busiest line in Britain.