The Glenbrook Tunnel is a heritage-listed single-track former railway tunnel and mustard gas storage facility and previously a mushroom farm located on the former Main Western Line at the Great Western Highway, Glenbrook, in the City of Blue Mountains local government area of New South Wales, Australia. The Department of Railways designed the tunnel and built it from 1891 to 1892. It is also known as Lapstone Hill tunnel and Former Glenbrook Railway and World War II Mustard Gas Storage Tunnel. The property is owned by Blue Mountains City Council and Land and Property Management Authority, an agency of the Government of New South Wales. It was added to the New South Wales State Heritage Register on 5 August 2011. The railway tunnel was originally part of the Glenbrook 1892 single-track deviation, which bypassed the Lapstone Zig Zag across the Blue Mountains. It is 634 metres; 693 yards long and is constructed in an 'S' shape with a gradient of 1:33.

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.