The Glenbrook Tunnel: Mustard Gas, Mushrooms, and Metal

If the walls of the Glenbrook Tunnel could talk, they'd whisper stories of steam, secrecy, and survival. Built in 1892 to replace the original Lapstone Zig Zag, this "S-shaped" tunnel was a nightmare for train crews. The steep 1:33 gradient meant steam engines often stalled inside, choking the drivers with smoke and soot. For more Blue Mountains heritage, explore Red Hands Cave or discover Newnes Glow Worm Tunnel.

By 1913, the trains were gone, but the tunnel's life was just beginning. Over the next century, it became a massive commercial mushroom farm and, more chillingly, a secret storage facility for mustard gas and chemical weapons during World War II.

Quick Stats

• Access: Restricted (Full opening expected 2026—viewing possible from portals).

• Location: Knapsack Reserve, Glenbrook/Lapstone.

• Length: 660 metres.

• Best For: Industrial heritage, WWII history, and moody photography.

How to Find the Tunnel Portals

The tunnel sits within the Knapsack Reserve, a treasure trove of rail history. Even when the tunnel itself is closed for work, you can explore the impressive "cuttings" (the deep trenches) that lead to it.

1. The Eastern Portal (Lapstone Side): Park at the dirt car park near the water tanks on Great Western Highway, Lapstone (just past the end of the M4). Follow the old rail cutting. You can walk right up to the massive bricked-up portal and see the original 1892 sandstone work.

2. The Western Portal (Glenbrook Side): Located near the end of Knapsack Street, Glenbrook. This end is more overgrown and atmospheric, often surrounded by the ruins of the old mushroom farm sheds.

A Dark History: WWII & Beyond

• The Gas Store: In 1942, the RAAF moved in. They laid a concrete floor and used the tunnel's cool, stable temperatures to store bulk chemical weapons, including mustard gas, to defend Sydney against potential invasion.

• The Mushroom Era: For nearly 100 years, the tunnel was home to one of Australia's most productive mushroom farms. The racks and humidity control systems used by the farmers are still visible in parts of the tunnel today.

• The "Fire's On" Connection: Famed Australian artist Arthur Streeton actually painted the construction of the nearby railway (the "Fire's On" painting), capturing the grit and danger of the men carving these tunnels into the sandstone.

Expert Tips for Your Visit

• Knapsack Bridge: If you're visiting the tunnel, you must walk the extra 10 minutes to the Knapsack Bridge (the Great Zig Zag Bridge). It's a massive sandstone viaduct that is one of the most photographed bridges in NSW.

• Flooding: The tunnel is built in an "S" shape and holds water. After heavy rain, the portals can be extremely muddy and the cuttings can become small creeks. Wear waterproof boots!

• Safety: Please respect all fences and council signage. Because of its WWII use, the tunnel has undergone extensive environmental testing to ensure it is safe for the public. Discover more NSW hidden gems in our complete guide.