A horseshoe curve is a reversing curve through a single tight curve in a railway or a road, through an angle of about 180 degrees or more. The U shape, or even slight balloon shape, of such a curve resembles a horseshoe, hence the name. On roads such curves, if tight enough, are typically called hairpin turns.
A horseshoe curve is a means to lengthen an ascending or descending grade and thereby reduce the maximum gradient. If the straight route between two points would be too steep to climb, a more circuitous route will increase the distance travelled, allowing the difference in altitude to be averaged over a longer track (or road) length. This is similar to the function of a spiral. However, a horseshoe curve does not involve the track crossing over itself, and the full horseshoe involves both relatively straight and tightly curved sections, while a spiral generally has a more uniform curvature. Obviously, a horseshoe also gives rise to a severe change in direction, while a spiral generally does not.
A horseshoe curve is sometimes used where the route bridges a deep gully. Deviating from a straight-line route along the edge of the gully may allow it to be crossed at a better location.
Horseshoe curves are common on railway lines in steeply graded or hilly country, where means must be found to achieve acceptable grades and minimize construction costs. 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.(Wikipedia)