Tunnels
There are two tunnels on the line, built between 1860 and 1867.
Mickleham Tunnel is midway between Leatherhead and Box Hill & Westhumble. It is 524 yards long and runs through the lower chalk of Norbury Park, entering the hillside immediately north of one of the three viaducts over the River Mole. Restrictions imposed by the landowner, Thomas Grissell, meant that vertical ventillation shafts could not be constructed and the tunnel portals were given lavish architectural treatment.
Betchworth Tunnel is 1/4 mile south of Dorking. It is 385 yards long with a maximum gradient of 1 in 80. It runs through the upper greensand of the Deepdene Ridge to the east of the town. Construction difficulties delayed the line's opening south of Dorking. It collapsed on 27 July 1887, remaining closed for over six months.
Read more about this topic: Sutton And Mole Valley Lines