The first Glenbrook deviation was the section of track from the first Knapsack Viaduct to Old Glenbrook Station in the Blue Mountains of New South Wales. It was constructed from 1891 to 1892 and replaced the Lapstone Zig Zag.
In 1890, it was proposed to bypass the troublesome Lapstone Zig Zag by utilising a tunnel. This proposal was soon approved and in March 1891, tenders were called for its construction. The line left Bottom Points and continued into a gully before entering the 2,165-foot-long (660 m) tunnel, on a continuous 1-in-33 gradient. After emerging just below the original alignment, the line curved to the left and ascended slightly before rejoining the original line at Glenbrook. The site of the original Glenbrook Station now lies to the right of the Great Western Highway, near the skate park.
However, the deviation soon proved to be somewhat of a disaster. Even if it did eliminate the Zig Zag which restricted train length, the problem was the design of the tunnel. The first problem was the climb, the tunnel being on a steep, continuous 1-in-33 gradient. The second and main problem was ventilation. The tunnel was single-track, which made the dimensions tight, and towards one end was a curve, which made the ventilation even worse. Passengers found themselves fighting off smoke and fumes from the locomotive. Trains would begin slipping halfway through the tunnel, forcing engine crews to retreat for air. In one incident of 1908, a retreating train met with another down goods train in a collision at the tunnel mouth.
In 1910, work started on a new deviation when the Blue Mountains was being duplicated at the time. The second Glenbrook deviation replaced the 1-in-33 deviation with a more gentle 1-in-60 ruling grade across Glenbrook Gorge.
In World War Two, the tunnel was used to store bombs and ammunition. After the war however, the tunnel found a useful purpose as a mushroom farm, which it currently serves as today.