Horseshoe curve

On roadways, particularly tight versions of such curves are typically called hairpin turns.

A horseshoe curve is a means to lengthen an ascending or descending grade and thereby reduce the maximum gradient.

Grade or gradient is defined as the rise divided by the run (length) or distance, so in principle such curves add to length for the same altitude gain, just as would a climbing spiral around one or more peaks, or a climbing traverse (cutting) wrapping around an end of a ridge.

As with spirals, the main limitation in laying out a horseshoe is keeping its radius as large as possible, as sharp curves limit train speed, and through increased friction, are harder on rails, requiring more frequent replacement of outer tracks.

Pennsylvania Alaska California Colorado Horseshoe curves were used extensively on the many narrow gauge railroads in the Colorado Rocky Mountains, now mostly abandoned,[4] for example: Idaho Maryland Montana Nevada New York Oregon Utah Washington British Columbia Jiangsu Shaanxi Sichuan Xinjiang Yunnan Gansu Qinghai