and twelve were injured. Tunnels There are two tunnels, built between 1860 and 1867. Mickleham Tunnel is midway between Leatherhead and Box Hill & Westhumble. It is 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 ventilation shafts could not be constructed and the tunnel portals were given lavish architectural treatment. Betchworth Tunnel is south of Dorking. It is long with a maximum gradient of 1 in 80 (1.25 per cent). It runs through the upper greensand of the Deepdene Ridge to the east of the town. Construction difficulties delayed the opening south of Dorking. It collapsed on 27 July 1887, remaining closed for over six months. See also Epsom Downs branch line Sutton Loop Line Chessington branch line South West Main Line Brighton Main Line New Guildford line North Downs Line Arun Valley line References External links Route Map and Timetable for Sutton & Mole Valley Line Rail transport in Surrey Railway lines in South East England Standard gauge railways in England Transport in the London Borough of Croydon Transport in Epsom and Ewell Transport in the Royal Borough of Kingston upon Thames Transport in the London Borough of Lambeth Transport in the London Borough of Merton Transport in the London Borough of Southwark Transport in the London Borough of Sutton