Can you summarize the following document?
        
  馬蹄彎是一種以加長上升或下降坡度,從而減少路線最大坡度的方法。坡度定義為上升距離除予長度或距離,原則上類似的彎道增加長度時,為保持坡度一樣,同樣的上升距離(海拔)也會隨之新增。就像展線環繞山丘或爬升越過環繞的山峰那樣。  如果一段兩點之間的路途因太陡而難以爬升,迂迴的線路會讓距離增加,使兩地水平面可以以長一點的軌道(或道路)達到平衡。馬蹄彎不像螺旋,它並不包括跨越它自己的軌道,且一個完整的馬蹄彎包括兩條相對的直線線段。  curve 拐點 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.

  

  The Cougal Spiral is a feature of the North Coast Railway in Australia that connects New South Wales with Queensland through Richmond Gap. The railway lines needs to climb at a steady ruling gradient from Kyogle to the summit at a tunnel at the border between the two states. The border is also at the watershed. The line is single tracked.
馬蹄彎是道路的一種爬行彎道,在拐點處翻轉兩次,最高可到達180度翻轉。