Metrology, the scientific study of measurements having origins in the early 19th century, formed from the Greek metron ‘measure’ + -logy.

When considering the distance between two adjacent rails used by either passenger or freight trains upon which they travel from point A to point B, you might conclude that by necessity there exists a single, standard definition for this dimension to be followed by all railroad lines operating in the USA, a “Standard” dimension. You would be wrong. While at the beginning of the Civil War the most often observed dimension between rails was 4’ 8.5”, there were more than 20 other different gauges ranging from 3 to 6 feet in use. 1 Congress would soon afterward settle on a “standard” dimension of 4’ 8.5” to be observed during construction of all transcontinental railways. The next questions one might consider are “what is an inch, a foot?” OK…one can get carried away with this train of thought…and ask themselves “What is a pound, a kilo…and what does that mean? If you study Metrology, the art and science of weights and measures, along with the evolution of standard definitions for each entity considered to be observed by all, you will soon have the answers to each of these, and hundreds more, derivations and definitions of weights and measures. But wait! Depending upon your fields of endeavor…you certainly may be interested in measurement standards outside the realm of just weights! How about Radio Frequencies, Time, Dimensions, Pressures, Stress, Flatness, Voltage and Currents, Resistance…just a small sampling of items the values of which are all considered by Metrologists around the world. The United States established the National Bureau of Standards (today known as NIST) in 1901 to address scientist’s and industrialist’s needs to have an authoritative source of standards for electricity, length, mass, temperature, light, and time, accompanied by a system whereby these standards could be transferred to the public. Uncertainty plagued U. S. markets at the turn of the 20th century. There were at least eight different definitions of a gallon, and four different definitions for a dimensional “foot” in use at the time. The NBS convened the first National Conference on Weights and Measures to write model laws, distribute uniform standards, and provide training for inspectors which heretofore had been poorly trained and working with outmoded equipment. ( Continued ) 1 https://www.truthorfiction.com/railwidth/