A horseshoe curve is a tight curve in a railway or a road, through an angle in excess of 180 degrees. A horseshoe curve is a means to lengthen the passage of an ascending or descending grade, so as to reduce the maximum gradient of ascent or descent. The horseshoe refers to the U shaped bypass of a straight route. The straight route would be too steep to climb, so instead the route takes a more circuitous route. In the case of roads, such curves, if tight enough, are typically called hairpin turns.
The route deviation thus undertaken greatly lengthens the actual distance travelled, and by so doing, it allows the difference in altitude to be averaged over a much longer track length. This is similar to the way a spiral works. However, a horseshoe curve does not involve the track crossing over itself, and the three main curves are only a part of a horseshoe, whereas a spiral is generally a continuous curvature. Another reason for a horseshoe curve is where the straight-ahead route involves bridging a deep gully. The deviation might allow the gully to be crossed at a more favourable location to one side.
Horseshoe curves are a common feature of railway lines in steeply graded or hilly country, where effective means have to be found to achieve an acceptable grade or construction cost of a line. Like spirals, the main limitation of the horseshoe is keeping the radius of the curvatures at or above a desirable minimum. Sharp curvatures will limit the speeds at which trains can run on a line.
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