Zig zag (railway)


A railway zig zag or switchback is a railway operation in which a train is required to switch its direction of travel to continue its journey. While this may be required purely from an operational standpoint, it is also ideal for 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. Some switchbacks do not come in pairs, and the train may then need to travel backwards for a considerable distance.
A location on railways constructed by using a zig-zag alignment at which trains must reverse direction to continue is a reversing station.
One of the best-known examples is the Darjeeling Himalayan Railway, a UNESCO World Heritage Site railway in India, which 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 for continuous grade, which must contend with the larger-scale geography of the hills to be surmounted.

Disadvantages

Zig zags suffer from a number of limitations:
  • The length of trains is limited to what will fit on the shortest stub track in the zig zag. For that reason, the Lithgow Zig Zag's stubs were extended at great expense in 1908. Even then, delays were such that the zig zag had eventually to be bypassed by a new route, which opened two years later.
  • Reversing a locomotive-hauled train not purposely equipped for push-pull operation without first running the engine around to the rear of the train can be hazardous; however, operating the train with two locomotives, one at each end, can mitigate the dangers.
  • The need to stop the train after each segment, throw the switch, and reverse means that progress through the zig zag is slow.
  • Passenger cars with transverse seating force riders to travel in reverse for at least part of the journey though that issue is largely solved by longitudinal seating on cars serving such routes.

    Hazards

If the wagons in a freight train are marshaled 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