Zig zag (railway)


A railway zig zag or switchback, is a method of climbing steep gradients with minimal need for tunnels and heavy earthworks. For a short distance, the direction of travel is reversed, before the original direction is resumed. Not all switchbacks come in pairs, in which case the train may need to travel backwards for a considerable distance.
A location on railways constructed by using a zig-zag alignment at which trains have to reverse direction in order to continue is a reversing station.
One of the best examples is the Darjeeling Himalayan Railway, a UNESCO World Heritage site railway in India, that has six full zig zags and three spirals.

Advantages

Zig zags tend to be cheaper to construct because the grades required are discontinuous. Civil engineers can generally find a series of shorter segments going back and forth up the side of a hill more easily and with less grading than they can a continuous grade which has to contend with the larger scale geography of the hills to be surmounted.

Disadvantages

Zig zags suffer from a number of limitations:
If the wagons in a freight train are marshalled poorly, with a light vehicle located between heavier ones, the move on the middle road of a zig zag can cause derailment of the light wagon.

Examples