A railway zig zag or switchback is a railway operation in which a train is required to switch its direction of travel in order to continue its journey. While this may be required purely from an operations 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.

The Darjeeling Himalayan Railway, also known as the DHR or the Toy Train, is a 610 mm gauge railway that runs between New Jalpaiguri and Darjeeling in the Indian state of West Bengal. Built between 1879 and 1881, it is about 88 km (55 mi) long. It climbs from about 100 m (330 ft) above sea level at New Jalpaiguri to about 2,200 m (7,200 ft) at Darjeeling, using six zig zags and three loops to gain altitude. Ghum station is situated at an altitude of 2,258 metres (7,407 ft). Six diesel locomotives handle most of the scheduled service, with daily tourist trains from Darjeeling to Ghum – India's highest railway station – and the steam-hauled Red Panda service from Darjeeling to Kurseong. Steam-enthusiast specials are hauled by vintage British-built B-Class steam locomotives. The railway's headquarters are at Kurseong.