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The first subway built in London in 1863, was London's Metropolitan Railway. It wasn't very long and its first tunnels weren't very deep. It linked Paddington Station to Farrington Street and took passengers into the centre of the City. It was soon necessary to work out a system to prevent clay from falling down onto the workers digging the tunnels.And the solution was a tube -shaped iron shield that advanced as the workers made progress.Nowadays many cities in the world have underground systems but the the London underground system is one of the largest in the world.