A tunnel is relatively long and narrow; the length is often much greater than twice the
diameter, although similar shorter excavations can be constructed, such as cross passages between tunnels.
The definition of what constitutes a tunnel can vary widely from source to source. For example, the definition of a road tunnel in the
United Kingdom is defined as "a subsurface highway structure enclosed for a length of 150 metres (490 ft) or more."
 In the United States, the
NFPA definition of a tunnel is "An underground structure with a design length greater than 23 m (75 ft) and a diameter greater than 1,800 millimetres (5.9 ft)."
In the UK, a pedestrian, cycle or animal tunnel beneath a road or railway is called a
subway, while an underground railway system is differently named in different cities, the "
Underground" or the "
London, the "
Glasgow, and the "
Newcastle. The place where a road, railway, canal or watercourse passes under a footpath, cycleway, or another road or railway is most commonly called a
bridge or, if passing under a canal, an
aqueduct. Where it is important to stress that it is passing underneath, it may be called an underpass, though the official term when passing under a railway is an
underbridge. A longer underpass containing a road, canal or railway is normally called a "tunnel", whether or not it passes under another item of infrastructure. An underpass of any length under a river is also usually called a "tunnel", whatever mode of transport it is for.
In the US, the term "subway" means an underground rapid transit system, and the term
pedestrian underpass is used for a passage beneath a barrier. Rail station platforms may be connected by pedestrian tunnels or