Definition of "overground"

overground¹

adjective
not comparable
1. Above the ground.
  Quotations
  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, page 57
2. Mainstream, not underground.
  Quotations
  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
noun
plural overgrounds
1. (chiefly London) A commuter rail or similar rail transport system, or a train in such systems, generally running above the ground.
  Quotations
  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
  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

overground²

verb
1. simple past and past participle of overgrind