a gauge

Originally, various track gauges were used in the United States. Some railways, primarily in the northeast, used standard gauge of 4 ft 8+1⁄2 in (1,435 mm); others used gauges ranging from 2 ft (610 mm) to 6 ft (1,829 mm). As a general rule, southern railroads were built to one or another broad gauge, mostly 5 ft (1,524 mm), while northern railroads that were not standard-gauge tended to be narrow-gauge. The Pacific Railroad Acts of 1863 specified standard gauge.
Notable exceptions were the 6 ft (1,829 mm) railroads that predominated in the first part of the 19th century in New York State, and the 5 ft 6 in (1,676 mm) lines centered on Portland, Maine. Problems began as soon as lines began to meet, and standard gauge was adopted in much of the northeastern U.S. Standard gauge had spread widely across the country by the late 19th century except in some parts of the South; it was adopted there in a two-day changeover on May 31-June 1, 1886.
Today, standard gauge is used almost everywhere in the U.S. Non-standard gauges remain in use only for some municipal and regional mass transit systems not requiring interchange of equipment.

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