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Clayton Tunnel dates from 1840 and was built for the London & Brighton Railway, which became the London, Brighton & South Coast Railway in 1846. At 1 mile and 500 yards it is the longest tunnel on the Brighton Line. The north portal is particularly memorable with its castellated turrets and house in the middle. It cannot have been too much fun to live there in steam days.
Clayton Tunnel was the scene on Sunday 25 August 1861, of the worst accident on the British railway system to that time. Two trains collided inside the tunnel, killing 23 and injuring 176 passengers. It is generally thought that Charles Dickens based his story "The Signal-Man" of December 1866 on this accident. His own experience at the Staplehurst rail crash may have inspired him to write the ghost story. An account of the Clayton Tunnel Disaster is at Link