Definition of "Underground"

Although some of the stations on ' The Branch ' have not, as yet, suffered any considerable structural alterations, all are now equipped with the familiar Underground type name boards, and as a result the imposing titles of G.N. days have been ruthlessly curtailed.

1940 October, “Notes and News: The High Barnet Branch”, in Railway Magazine, page 569

Heathrow has a 28% share of passengers arriving by rail, two-thirds of whom come by the Underground.

2019 October, John Glover, “Heathrow rail expansion”, in Modern Railways, page 71