Colour-light signals, for both day and night work, came into use at a much later date, and their first use on any scale in this country was on the London underground railways, where the principle adopted was largely American in origin. But although light signalling is credited to America, one of the first instances of its use was an experimental installation at Herne Hill, although it did not find favour in this country for overground working until some fifty years later.1955 April, R. H. Warring, “Some Early Railway Signals”, in Railway Magazine, pages 231-232:
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We have been concentrating on the City because that was the initial target for builders of both overground and underground railways, who were like artists going over and over the same piece of canvas.2012, Andrew Martin, Underground Overground: A passenger's history of the Tube, Profile Books, →ISBN, page 57:
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Mainstream, not underground.
They have plans to carry overground books on gay liberation, and a lot of books from small presses.1974 April 6, “H.C.H.S. Bookstore”, in Gay Community News, page 1:
overground
noun
Etymology From over- + ground.
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London A commuter rail or similar rail transport system, or a train in such systems, generally running through an urban area largely above ground and faster than an underground or metro with fewer stops.
The overground is ten minutes walk through the gloom. Anna finds a seat in the third carriage she tries.2013, Tobias Hill, The Cryptographer:
Today dozens more systems are being built – or at least being planned with twelve in China alone – and that's not counting the overgrounds and the tramways!2014, Nicholas Faith, The World the Railways Made:
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TfL has released an official map showing how the new Elizabeth Line will slot in with the existing stations, Tube lines and overgrounds.2022 May 19, Lucy Aplin, “New Tube map: How the London Underground map looks with the Elizabeth Line ahead of Crossrail opening”, in i: