In 1871, railroad tracks in the United States had 23 different gauges, ranging from three to six feet. Railroads in the South used a standard gauge of five feet—three inches wider than the standard adopted by a convention in 1886. So, on May 31st and June 1st of 1886, 11,000 miles of tracks were nudged a little closer together.
A few days before May 31, all roads began clearing cars from their lines and reducing the gauge of all areas of track that could be freed of cars and engines.
Finally, in the early morning hours of May 31, the concentrated work began. Men worked in crews of various sizes charged with various goa[l]s—some given specific mileages to cover, others under instructions to begin at a specified point and work in a specified direction until they met another crew working toward them.
In less than three days, standard-gauge trains were serving the South. "The work was done economically," an article in the Journal of the Association of Engineering Societies pointed out, "and so quietly that the public hardly realized it was in progress. To the casual observer it was an every-day transaction." [Link, via MetaFilter]This reminds me of another great moment in standardization, also prompted by the railroads: November 18, 1883, The Day of Two Noons.