High Quality Content by WIKIPEDIA articles! A spiral (sometimes called a spiral loop or just loop) is a technique employed by railways to ascend steep hills. A railway spiral rises on a steady curve until it has completed a loop, passing over itself as it gains height, allowing the railway to gain vertical elevation in a relatively short horizontal distance. It is an alternative to a zig-zag, and avoids the need for the trains to stop and reverse direction while ascending. If the train is of sufficient length, it is possible to view the train looping onto itself. A spiral loop should not be confused with the transition spiral or spiral easement used to provide a transition from a tangent into a horizontal circular curve. Spiral easement is used to avoid abrupt changes in the centripetal acceleration experienced by a railroad vehicle and the passengers in the vehicle approaching the horizontal circular curve and to prevent abrupt forces and discomfort. These curves are also used in highway engineering. A similar feature to railway spirals in road design is the pigtail bridge.

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