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A horseshoe curve is a 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 bears an obvious resemblance to a horseshoe, and hence the name. In the case of roads, such curves, if tight enough, are typically called hairpin turns.
A horseshoe curve is a means to lengthen the passage of an ascending or descending grade and thereby reduce the maximum gradient of ascent or descent. In other words, if the straight route between two points would be too steep to climb, a more circuitous route is chosen in order to increase the actual distance travelled, thereby 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 also used where the route between two points involves bridging a deep gully. Deviating from a straight-line route along the edge of the gully may allow it to be crossed at a more favourable location.
Horseshoe curves are a common feature of railway lines in steeply graded or hilly country, where effective 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 the radii of curvature at or above a desirable minimum, as sharp curvatures will limit the speeds at which trains can run on the line.
Examples of horseshoe curves 
North America 
- Foss Creek, between Skykomish, Washington and the Cascade Tunnel.
- Chorro, California on the grade from San Luis Obispo to Cuesta Pass.
- The Cantara Loops between Dunsmuir, California and Mount Shasta, California. Map
- East of Oakridge, Oregon on the Cascade Line.
- Horseshoe Curve (Pennsylvania), at Kittanning Gap.
- Notch Hill, on CP's Shuswap Sub near Salmon Arm, British Columbia.
- Kamaishi Line of East Japan Railway Company in Japan, has a horseshoe curve from Kamiarisu Station down to Rikuchū Ōhashi Station ("down" on the elevation and registration of direction of the line).
- Nanning–Kunming Railway in China, inside Yiliang County, located in the east of Yiliang Town.
- Picton railway station, New South Wales, turns back on itself at about 225 degrees.
- The Raurimu Spiral in New Zealand has a horseshoe curve as the first part of the climb.
- Dovrebanen, the main line of the Norwegian railway network, has a horseshoe from Dombås and up to the Dovre plateau.
- Flåmsbana, Norway, has a double horseshoe, one inside a tunnel, one in the open, few kilometres below top station.
- Grybów, Poland has a horseshoe curve 2,5 km west of the town.
- Kalisz, Poland has a double horseshoe curve leading the tracks from a flat plateau down to the valley of the Prosna river.
- Between Jelenia Góra and Szklarska Poręba in Poland there is a five-times, elongated horseshoe curve (50°51′19"N, 15°34′17"E). Map
- The Rhein-Ruhr S-Bahn in Germany has a horseshoe curve in Neviges, Velbert on the route between Essen and Wuppertal, known as the Prince William railway.
- The horseshoe curve on the West Highland Line in Scotland between Upper Tyndrum and Bridge of Orchy was built because the engineers of the railway couldn't afford to build a viaduct crossing the remote valley.