"filling in" meaning in English

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          "ref": "1955 June, “Notes and News: Locomotive Notes: London Midland Region”, in Railway Magazine, page 439:",
          "text": "It was to assist in the filling-in of the tunnel on the disused Patricroft-Clifton Junction line, which was the scene of a disaster in 1953 when part, below some houses in Swinton, caved in.",
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