I just received this handy reference book today, and notice that distances are given in miles and ‘chains’. Are chains a particularly rail orientated unit of measurement? There are 80 in a mile, and 1 chain is about 20 metres. This got me wondering if the route measurements in the HUD were miles and chains too, but you might see something like ‘0.93’ to the next signal, so if there are 80 chains in a mile, it can’t be that? What does 0.93 mean? Is it literally 0.93 of a mile, a metric percentage fraction of an imperial unit? The manual doesn’t seem to say...
Chains is an old nautical measurement adopted by the railway engineers for track length and the radius for curves. Not sure if chains is still used. I stand to be corrected.
Chains are very much still used, in fact if you go onto www.realtimetrains.com and look up any service that has the length included they're also in miles and chains even though it puts it like a decimal (ie separated by a point) It is an odd thing 1/80 of a mile, but it it's a workable system I don't see much need to change it bar human comprehension. Machines can count in 80s as easily as they can any other number
If I remember a chain is about 20 metres, I can cope! Here’s a map showing distances in miles and chains.
Isn't it the length of a cricket pitch? 22 yards - 1/10 of a furlong or 1 furrow? The chain is the nautical variant of the old Saxon agricultural measurements. From: https://www.espncricinfo.com/story/the-measurements-of-cricket-89685 In 1610 Edmund Gunter, an Oxford trained mathematician, now Professor of Astronomy at Gresham College, London, invented as an instrument of measurement the chain, taking its length from the breadth of the furrow and dividing it into 100 links of 7.92 inches each (i.e. 4 perches [not 40 as stated by the Encyclopaedia Britannica, 15th ed., vol. 19, p. 729, which is the length of the furrow]; By 1661 use of this chain had become sufficiently popular for the word to be used to designate the measurement itself}. This chain became the common measuring tool for land surveyors. We do not know when cricketers first wished to standardize their pitch, but in the eighteenth and nineteenth centuries at least pitches were often physically marked out with the use of Gunter's chain.
In the UK chains were used to describe the radius of curves, and the smallest curve permitted was often written on the side of long wheelbase rolling stock. Modern practice is to use metres, I don't know when the change took place.