Definition of "tunnel"

In 1865 an outfit called the East London Railway Company bought the Brunel tunnel for £800,000, and in 1869 they opened a railway through it.

2012, Andrew Martin, Underground Overground: A passenger's history of the Tube, Profile Books, page 90

But very soon he grew to like it, for the Boy used to talk to him, and made nice tunnels for him under the bedclothes that he said were like the burrows the real rabbits lived in.

1921 June, Margery Williams, “The Velveteen Rabbit: Or How Toys Become Real”, in Harper’s Bazar, volume LVI, number 6 (2504 overall), New York, N.Y.: International Magazine Company

There are more than 1,500 railway tunnels in Britain and the majority are still in use, carrying working tracks beneath Britain's most inconvenient geographic features.

2020 August 26, Tim Dunn, “Great railway bores of our time!”, in Rail, page 42

And one great chimney, whose long tonnell thence, / The smoke forth threw

1590, Edmund Spenser, “Book II, Canto IX”, in The Faerie Queene. […], London: […] [John Wolfe] for William Ponsonbie, stanza 29

Especially in the Eden Valley, trees create what is almost a green tunnel (particularly in summer).

2021 October 20, Mark Rand, “S&C: a line fit for tourists... and everyone?”, in RAIL, number 942, page 43

The 1955 Act gave powers for compulsory acquisition of "easements", or permission to tunnel beneath dwelling houses instead of, as had previously been necessary, following approximately the course of surface roads.

1962 October, “London gets its Victoria tube”, in Modern Railways, page 258

The 6.5km route is agreed from a junction with the relief lines of the Great Western main line to the west of Slough, the new link would tunnel under the M25 to reach Heathrow's Terminal 5 station, where space has been set aside to accommodate services from the west.

2019 October, Ruth Bagley tells James Abbott, “Crunch time for Heathrow western link”, in Modern Railways, page 74