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.1955 June, “Notes and News: Locomotive Notes: London Midland Region”, in Railway Magazine, page 439:
2
To cause to collapse inward or downward.
He caved in the side of the barrel with a single well-placed kick.
3
Figuratively To relent; to grant approval against one's initial will.
After he asked me a few times, I finally caved in and had a slice of cake.
Eventually the NUR overplayed its hands with an all-out strike. And when Peter Parker, the then-chairman of BR, who was well regarded among his staff, called their bluff by threatening to close down the entire network, they caved in.2022 December 14, Christian Wolmar, “Productivity should play no part in pay negotiations”, in RAIL, number 972, page 46:
cave-in
noun
Etymology Deverbal from cave in.
1
The act of something collapsing or caving in.
We were warned away from the rickety old mine because of the danger of cave-ins.
2
The location where something has caved in.
We couldn't get past the cave-in blocking the tunnel.