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In the U. S., chaining is a method by which railroads precisely measure and specify locations along the line. It measures distances from a fixed point, called chaining zero, following the twists and turns of the railroad line, so that the distance described is understood to be the "railroad distance," not the distance by the most direct route ("as the crow flies").
At least three different methods of measuring distance on railroads are in use in the United States: the milepost or mileage system, where distances are usually measured in miles and hundredths, the engineer's chain of 100 feet (~30.5 m) in length, and the Gunter's chain of 66 feet (1/80 of a mile).
Chaining zero is a fixed point from which the chaining is measured on a particular chaining line. Assuming that 100-foot chains are used, a chaining number at a specific location (called a chaining station) on a line of 243, say, identifies that the point is 24,300 feet (~4.6 miles or ~7.4 km) from chaining zero, usually measured along the center line of the railroad.
Chaining lines are routes on physical railroad lines that are usually described by one or two letters for the purpose of identifying locations on those lines.
Chaining lines are not necessarily the same as the physical lines they run on. One physical line may have several chaining letters, and one chaining line may cover several physical lines.
The letters assigned to a chaining line have nothing whatever to do with the letters displayed on trains and public maps and timetables.
Each specific location along a line is known as a chaining station, and is identified by a number unique to that chaining line. The precision of the location depends on its usage. On engineering maps used in the New York City Subway, the location of such features as curves, switches, crossings, stations and platforms are ordinarily specified to a precision of one foot (30.48 cm). This is expressed as [chains plus feet]; a chaining station located 1,470 feet from chaining zero would be described as 14+70. For greater precision, or where style or protocol requires it, unit of less than a foot may be described. The Brooklyn Rapid Transit Company used [chains plus feet point hundredths] without trailing zeros: thus a map location designated as P.S. 14+70.25 would indicate that the Point of a Switch at that location is 1,470 and one quarter feet (1,470 feet and 3 inches) from chaining zero.