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1,830 Yards. Merstham Tunnel - London, Brighton and South Coast Railway
The original tunnel through the North Downs was constructed under the supervision of the engineer John Urpeth Rastrick for the London and Brighton Railway (L&BR) between 1839 and its opening on 12 July 1841. It was cut through chalk, using twelve vertical shafts, up which the spoil was raised to the surface in skiffs by means of two-horse gins. The tunnel was originally intended to be 2013 yards (1841 metres) in length, but during the course of construction it was realigned and became 1,830 yards. The new tunnel was finished with brick portals, whitewashed and lit by gas lamps supplied from a small gas works south of Merstham station.
The two-mile railway cutting to the north of the tunnel is one of the largest in Europe and is 100 feet (30.5 metres) deep at the northern entrance to the tunnel. Its construction involved the removal of more than a million cubic yards of chalk.
Although the tunnel was built by the L&BR, it was on a section of line between Croydon and Redhill that was to be shared with the South Eastern Railway (SER), when they commenced services on their route to Dover in 1842.
On 16 July 1844 the SER refunded half of the construction cost of the joint line and took ownership of the section between Purley and Redhill, including the Merstham Tunnel.