Going metric - miles, furlongs, chains, rods, link

A chain is a unit of length. It measures 66 feet, or 22 yards, or 100 links, or 4 rods. There are 10 chains in a furlong, and 80 chains in one statute mile. An acre is the area of 10 square chains (that is, an area of one chain by one furlong). The chain has been used for several centuries in Britain and in some other countries influenced by British practice.
In Britain, although the chain is no longer used for practical survey work, it survives 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 entirely linear in topology, the "mileage" is sufficient to identify a place uniquely on any given route.
In this case, the bridge may be said to be "at" 48m 69ch, meaning that it is at the location 48 miles and 69 chains from the origin - in this case London Victoria Station.