Ahorseshoe curve is a class of climbing curve in aroadbed that reverses turn direction (inflection) twice on either side of a single tight curve that varies through an angle of about 180 degrees or more.
Such curves are more commonly found in arailway line but are also used inroads. The characteristic U shape, or even slight balloon shape, of such a curve resembles a horseshoe. Onroadways, particularly tight versions of such curves are typically calledhairpin turns.
A horseshoe curve is a means to lengthen an ascending or descendinggrade 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 aclimbing spiral around one or more peaks, or a climbingtraverse (cutting) wrapping around an end of a ridge.
If the straight route between two points is too steep to climb, a more circuitous route will increase the distance traveled, allowing the difference in altitude to be averaged over a longer track (or road) length. Unlike a spiral, a horseshoe curve does not involve the track crossing over itself, and the full horseshoe involves both relatively straight sections, curvedeflections in both directions and tightly curved segment; while a spiral generally has a more uniform curvature. Obviously, a horseshoe also gives rise to a severe change in direction requiring another corrective curve to regain displacement in the overall direction of travel, 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, and through increased friction, are harder on rails, requiring more frequent replacement of outer tracks.
TheFlåm Line, Norway, has a double horseshoe, one inside a tunnel, one in the open, few kilometres below top station, standard gauge, single track.
TheRauma Line, Norway, has a double horseshoe through the steep and narrow valley at Verma, one inside a tunnel and one that includes theKylling Bridge, standard gauge, single track.
InSlovakia there is a significant number of horseshoe curves on theBanská Bystrica toTurčianske Teplice railway track and on the railway fromZvolen toTurčianske Teplice. More than 20 tunnels and couple of horseshoe curves were built to overcome rough terrain and elevation differences.
Newcastle Quayside branch, a goods-only railway from the main line to the river quayside, through a steeply descending horseshoe tunnel.
The horseshoe curve on theWest Highland Line inScotland 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.
Mance Curve, between Meyersdale and Hyndman, Pennsylvania, alongSand Patch Grade. Long operated by the Baltimore & Ohio Railroad, it is today part ofCSX Transportation'sKeystone Subdivision) used by freight trains and Amtrak'sCapitol Limited (Chicago-Washington, D.C.) passenger train
In the Loop District of theAlaska Railroad betweenmileposts 48 and 51 northeast ofSeward, Alaska, there was a horseshoe and aspiral, both on an extensive range of timbertrestles up to 106 feet high. In 1951, a new route removed the original horseshoe, the spiral and all the trestles but added a new horseshoe at milepost 48.[2]
Also on the Trans-Iranian in theZagros Mountains, a pair of tightly linked horseshoes forms a "figure of 8" in which one loop almost completely contains the small city ofSepiddasht, Lorestan. The other loop is almost entirely within a tunnel, with both itsportals nearly side-by-side but several meters different in elevation. Both loops rotate about 250 degrees each.
TheCougal Spiral is a feature of theNorth Coast Railway in Australia that connects New South Wales with Queensland throughRichmond Gap. The railway line climbs at a steady ruling gradient from Kyogle to the summit at a tunnel at the border between the two states.
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^Prince, B.D.The Alaska Railroad in Pictures 1914-1964, Ken Wray's Print Shop, Anchorage, 1964
^Crump, Spencer (1998).Redwoods, Iron Horses, and the Pacific (Fifth ed.). Fort Bragg, California: California Western Railroad. p. 60.ISBN0-918376-12-2.
^Ormes, R.M.Tracking Ghost Railroads in Colorado, Century One Press 1975 (Contains extensive local maps identifying railroad names and dates of service).
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^"铁影 – 宏伟壮丽的展线群——成昆铁路运转记其四" [Iron Shadow – The magnificent railway line group - the fourth chapter of the operation of Chengdu-Kunming Railway] (in Chinese). 2017-07-17. Retrieved2024-08-08.
