3 Answers

Its built that way so the trains will not fall off of them,

To those of whom it may interest.

Track gauge in the United States

Originally, various gauges were used in the United States and Canada. Some railways, primarily in the northeast, used standard gauge; others used gauges ranging from 4 ft (1,219 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. Problems began as soon as lines began to meet and in much of the north-eastern United States, standard gauge was adopted.

Broad gauges

6 ft (1,829 mm) gauge

The Erie Railroad was originally 6 ft (1,829 mm) gauge.

5 ft 6 in (1,676 mm) gauge

The Indian gauge, also called "Provincial gauge" or "Texas gauge" of 5 ft 6 in (1,676 mm), was required by Texas law until 1875, and was used by the New Orleans, Opelousas and Great Western Railroad (NOO&GW) until 1872, the Texas and New Orleans Railroad until 1876, and the Maine Central Railroad until 1871 (and is currently used by the Bay Area Rapid Transit System).

5 ft (1,524 mm) gauge

In most of the southern states, the 5 ft (1,524 mm) gauge was preferred (a broad gauge which later was adopted by Russia for its new railroad and became known as Russian gauge). This configuration allowed for wider rolling stock that could more efficiently accommodate cotton bales, the most commonly transported good in the South at the time.

4 ft 10 in (1,473 mm)

Most of the original track in Ohio was built in 4 ft 10 in (1,473 mm) gauge, the "Ohio Gauge".

Non-standard broad gauge systems in the U.S.

In modern uses certain isolated occurrences of non-standard gauges can still be found, such as the 5 ft 21⁄2 in (1,588 mm) Pennsylvania trolley gauge tracks of the Philadelphia streetcars, the Philadelphia Market-Frankford subway cars, Pittsburgh Light Rail, and the New Orleans streetcars. The Bay Area Rapid Transit system in the San Francisco Bay Area, was built to 5 ft 6 in (1,676 mm) Indian gauge.

Towards standardisation

In the early days of rail transport in the United States, railroads tended to be built out of coastal cities into the hinterland, and systems did not connect. Each builder was free to choose its own gauge, although the availability of British-built locomotives encouraged some railroads to be built to standard gauge. When American railroads' track extended to the point that they began to interconnect, it became clear that a single nationwide gauge would be a good idea. Where different gauges meet, there is a "break of gauge". To overcome this issue, special compromise cars were able to run 4 ft 10 in (1,473 mm) and standard gauge track. Another application was the Ramsey Car Transfer Apparatus.

Gauge war

In Erie, Pennsylvania, the 6 ft (1,829 mm) Erie Railroad terminated while adjacent railroads used 4 ft 10 in (1,473 mm) gauge, also known as "Ohio gauge." This led to the Erie Gauge War in 1853-54 when the Erie mayor and citizens temporarily prevented a gauge standardization, as there would then be less trans-shipping work and through passengers would no longer have to stopover at Erie.

Pacific Railway Act of 1863

Break of gauge would prove to be a nightmare during the American Civil War (1861–1865), often hindering the Confederacy's ability to move goods efficiently over long distances. The Pacific Railway Act of March 3, 1863, specified that the federally funded transcontinental railroad was to use standard gauge and helped to further popularize it among American railroads, although the standard gauge was already in use on many other lines prior to 1863.

Pressure for standardisation

Following the Civil War, trade between the South and North grew and the break of gauge became a major economic nuisance. Competitive pressures had forced all the Canadian railways to convert to standard gauge by 1880, and Illinois Central converted its south line to New Orleans to standard gauge in 1881, putting pressure on the southern railways.

Unification to standard gauge on May 31, 1886

In 1886, the southern railroads agreed to coordinate changing gauge on all their tracks. After considerable debate and planning, most of the southern rail network was converted from 5 ft (1,524 mm) gauge to 4 ft 9 in (1,448 mm) gauge, then the standard of the Pennsylvania Railroad, over two remarkable days beginning on Monday, May 31, 1886. Over a period of 36 hours, tens of thousands of workers pulled the spikes from the west rail of all the broad gauge lines in the South, moved them 3 in (76 mm) east and spiked them back in place. The new gauge was close enough that standard gauge equipment could run on it without problem. By June 1886, all major railroads in North America were using approximately the same gauge. The final conversion to true standard gauge took place gradually as track was maintained. Now, the only broad-gauge rail systems in the United States are some city transit systems.

Narrow gauge systems in the U.S.

The San Francisco cable cars use a narrow gauge of 3 ft 6 in (1,067 mm), as did the Los Angeles Railway and the San Diego Electric Railway until 1898, and that gauge is still widely used in the U.S. mining industry.

3 ft (914 mm) gauge railways became the dominant narrow gauge throughout the United States.

2 ft (610 mm) gauge also had various installations.

Source : Wikipedia

The U.S. standard railroad gauge (distance between the rails) is four feet, eight and a half inches.

Why was that gauge used? Because that's the way they built them in England, and English expatriates built the U.S. railroads.

The gauge is about the same as carts which was based on the width of the butt end of a horse. Early railways were horse-drawn so the same gauge was used. It is said that ruts worn in the paving stones in ancient Rome were also the same distance apart.

There are other gauges such as Brunel's broad gauge ( 7ft 1/4 inch) and the Irish gauge 5ft3in, and many more.