Editing Chain (unit) (section)

===United Kingdom===
[[Image:Bridge miles and chains.jpg|thumb|Location marker painted on a British railway bridge, showing 112 miles and 63 chains; photograph taken August 2007]]
In the United Kingdom, the chain is no longer used for practical survey work.<ref>Plane and Geodetic Surveying, A.L. Johnson (SPON)</ref> However, it is still used on the railways as a location identifier. When railways were designed, the location of features such as bridges and stations was indicated by a cumulative longitudinal "mileage", using miles and chains, from a zero point at the origin or headquarters of the railway, or the originating junction of a new branch line. Since railways are linear in topology, the "mileage" or "chainage" is sufficient to identify a place uniquely on any given route. Thus, a given bridge location may be indicated as 112 miles and 63 chains (181.51&nbsp;km) from the origin. In the case of the photograph, the bridge is near [[Keynsham]], which is that distance from [[London Paddington station]]. The indication "MLN" after the mileage is the [[Engineer's Line Reference]] describing the route as the [[Great Western Main Line]], which is needed to uniquely determine the bridge, as there may be points at 112 miles 63 chains on other routes.

On new railway lines built in the United Kingdom such as [[High Speed 1]], the position along the alignment is still referred to as "chainage", although the value is now expressed in metres.<ref>[http://assets.dft.gov.uk/publications/hs2-maps-20120110/hs2arp00drrw05022issue4.pdf HS2 proposed alignment with chainages expressed in metres]</ref>