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.

Thornleigh is a suburb in the Upper North Shore region of Sydney in the state of New South Wales, Australia. Thornleigh is located 22 kilometres north-west of the Sydney central business district in the local government area of Hornsby Shire. The source of the Lane Cove River is located in Thornleigh.