The Lithgow Zig Zag is a heritage-listed former zig zag railway line built near Lithgow on the Great Western Line of New South Wales in Australia. The zig zag line operated between 1869 and 1910, to overcome an otherwise insurmountable climb and descent on the western side of the Blue Mountains. It was designed by John Whitton and built from 1863 to 1869 by Patrick Higgins as contractor. It is also known as the Great Zig Zag Railway and Reserves and Zig Zag Railway. The property is owned by Department of Planning and Infrastructure. It was added to the New South Wales State Heritage Register on 2 April 1999.

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.