was electrified in 1940. From this point on London Transport trains used only one of the tracks on the Mill Hill branch and the other track was soon lifted. Freight on the line lasted until 1964 when the line west of Mill Hill East was abandoned, leaving the present arrangement. Length, span and architecture The viaduct's 13 segmental arches carry it across the valley of the Dollis Brook, and over Dollis Road. Each arch spans at the springer level, and is based on tapered piers. In each pier there is an opening with an arched soffit and a dished invert. The viaduct is made from brick. It is the highest point on both the Northern line and the highest point on the London Underground above ground level, reaching nearly . It is not, however, the highest altitude on the Underground system, which is near Amersham (Metropolitan line) in the Chiltern Hills, at above sea level. Around the viaduct Below the viaduct can be found the Dollis Valley Greenwalk, a long-distance footpath between Moat Mount Open Space in Mill Hill and Hampstead Heath. This is designed to link many green spaces and wildlife corridors along the way and is approximately long. The geology nearby is of London Clay with small amounts of other rock types. An examination in the late 19th century of the railway cutting between Finchley and Hendon station found a large extent of glacial beds and fossils, and these were thought to extend as far northwest as the viaduct. See also List of bridges in London References External links Pictures of the viaduct Buildings and structures in the London Borough of Barnet Railway viaducts in London Finchley Transport infrastructure completed in 1867 19th-century architecture in the United Kingdom