The Lapstone Zig Zag was a zig zag railway built between Emu Plains and Blaxland stations on the Main Western Line of New South Wales in Australia. Constructed between 1863 and 1865 to overcome an otherwise insurmountable climb up the eastern side of the Blue Mountains, the zig zag and associated Knapsack Viaduct, a sandstone arch viaduct, were designed by John Whitton, Engineer-in-Charge of New South Wales Government Railways, and were built by William Watkins. The zig zag was listed on the Blue Mountains local government heritage register on 27 December 1991; while the adjacent Knapsack Viaduct was listed on the New South Wales Heritage Database on 2 April 1999. The Lapstone Zig Zag was the world-first Zig Zag constructed on any main-line railway.

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.