Gradient

Gradient means the inclination of a road or railway to the horizontal. It is measured in terms of the vertical height ascended or descended for a given length traversed along the slope. If the fall along four miles of road be 44 feet, the gradient is said to be 11 feet per mile or 1 in 480. The choice of gradient is very important in the economy of railway design. The route is generally so planned that the amount of excavation in the formation of the railway shall be roughly equal to the amount of embankment necessary where the route is to pass over low-lying levels. This condition usually imposes a limit on the gradient, though not necessarily so. Except in special cases where steep gradients are unavoidable, they are not allowed to exceed 1 in 60, the prevailing gradient on many good English lines being 16 feet to the mile or 1 in 330. If a high level has to be traversed, it is preferable to effect it in a series of short steep gradients with level interspaces between them rather than in a single uniform gradient upwards throughout the whole length. The locomotives are thus enabled to recover steam at each level stretch. Specially steep gradients require locomotives of unusual' construction. Appliances for increasing the adhesive power on the rails are used, or else a pinion works in a toothed rack laid along the line of railway, so that retrograde motion can only occur when the teeth break. In the Zermatt railway in Switzerland the maximum gradient on the adhesive sections is 1 in 22, and on the rack and pinion sections 1 in 8.