flyunder
nounUK /flʌɪˈʌndə/
Etymology From fly + under, to contrast with earlier flyover.
1
A road or rail underpass.
- An essential part of the Tilbury line electrification scheme is the remodelling to be carried out by the Eastern Region in the Barking area, which among other major works includes a new single-track flyover and a new single-track flyunder to carry the westbound District Line over and under the Fenchurch Street-Southend line, besides a main-line flyover and extensive permanent way and platform alterations.1956 July, “New Works on District Line, L.T.”, in Railway Magazine, page 485: