By the second half of the 19th Century, in the colony of New South Wales, the Great Southern Railway that ran from Sydney to Picton had become totally inadequate. Settlement was spreading south beyond Goulburn and valuable natural resources such as marble and sandstone, coal and shale, as well as timber and farm produce had to be carted to Sydney by bullock wagons over difficult terrain on roads that were at times barely passable. For people travelling the journey was tedious, slow, uncomfortable and often downright dangerous.
In 1863 the Colonial government decided to extend the railway southwards, to include Mittagong, Bowral, Moss Vale and Marulan, and to then terminate at Goulburn.
Under the direction of Engineer-in-Chief John Whitton, the work was completed in 1869, giving a total distance from Sydney to Goulburn of approximately 140 miles (224 km).
Between Mittagong and Bowral there lies Mount Gibralter. Thought to be a section of the rim of an ancient volcano, “the Gib” (as it is known to locals) is a spectacular bluff that rises some 868 metres and towers over the surrounding area. As there was no more accessible way around, the railway engineers required the boring of a deep tunnel through one side of the Gib, so as to provide a workable gradient for the railway line.
AT THE TIME, AUSTRALIA’S GREATEST ENGINEERING FEAT
Whitton’s deputy engineer, George Cowderey who, like his master, had built railways in England, was appointed to supervise the tunnel work, which up to that time constituted Australia’s greatest engineering feat. The contract for constructing the tunnel between Mittagong and Bowral for the single line railway, awarded to the firm of Wolf and Humphreys under the title of contract number four, started in 1863.
Shafts were sunk at different points of the course to be taken. The three deepest shafts were 156 (48 metres), 136 and 150 feet deep, respectively. Drives were commenced at the foot of each shaft. As it was dug and blasted, the shale and sandstone rubble was loaded into skips. The skips were hauled to the top and then the rubble was removed and dumped beside the line on the Mittagong side. Fresh air was pumped into the working area during excavation.
This excavation work took two years to complete and another year was required to line the tunnel with masonry blocks and brick. Hewn through hard shale and sandstone, the tunnel is over half a kilometre in length and up to 70 metres below ground level. Hundreds of men were engaged in the task; they lived in a tent encampment beyond the northern portal.
The work was completed in 1866, one year before the railway arrived in Mittagong in 1867. The well preserved ‘1866’ date-stone over the keystone at the northern end of the tunnel is still clearly visible today.
This tunnel served as a single-line railway tunnel until the Railways Department began what was known as the duplication, or construction of double lines in 1918, when a new double-line tunnel was to be constructed beside the original one.
When the new tunnel to carry the double lines for the main Sydney to Melbourne line was opened in December 1919, the original tunnel was abandoned.
Source of landfill for WW11 aerodrome at Mittagong During World War 11, when Australia appeared to be threatened with Japanese invasion, the authorities decided to upgrade the old Mittagong airstrip for possible use by the air force. The actual landing strip was lengthened and tar-sealed and several hangers with camouflage roof and sides wer e built on the site.
All this construction converting a small grass landing strip to a much longer aerodrome required a large amount of good landfill for raising levels, forming drains etc., and it was decided to use the enormous bank of rock and shale fill from the original excavations of the two Mount Gibraltar railway tunnels. This fill was transported by trucks running day and night to the aerodrome site.
The old abandoned tunnel was then recognised as a comparatively safe storage for explosives during possible enemy bombing.
STORAGE FOR EXPLOSIVES DURING WW11
In mid-1942 the unused railway tunnel was taken over by the Royal Australian Air Force for the storage of explosives.
The NSW Department of Railways had previously given permission to the Bowral Council for townspeople to use the tunnel as a bomb shelter. However, this permission was withdrawn to facilitate its use by the Air Force.
No.17 Replenishing Centre was formed as a completely self-accounting unit in February 1943. Its purpose was for the receipt, storage and replenishing of explosives used by operational units located in NSW.
Heavy doors were built on both ends of the old tunnel and stocks were immediately brought in. By the end of February 1943, 774 tonnes of explosives were in stock. A base camp was located nearby on the five-hectare property called “Mount Gibraltar” hired from the Sydney City Mission.
During May 1943 the area had 16 days of continuous rain with 178 millimetres falling on 17 May and 75mm falling in an hour and a half on 20 May. This heavy shower flooded the tunnel to a depth of between 300 and 380 mm, but very little damage was done to the stock.
Members of the unit rendered great assistance to the local community by helping fight the numerous bushfires which broke out throughout the district. At the end of the war the unit had a total strength of 38 men.
In November 1945 an Operational Order was received directing the unit to cease functioning and commence disbandment procedures. The Replenishing Centre finally disbanded on 1 December 1945. The site then came under the control of the RAAF’s No.1 Central Reserve as a detachment. The stocks of explosives left behind remained stored in the tunnel and six airmen stayed on as caretakers.
Although planning to vacate began at the end of 1951 it was not until 11 September 1953 that all stocks of explosives and equipment were eventually cleared. On this date the site closed and the last buildings were handed back to the Sydney City Mission.
MUSHROOM CULTIVATION IN POST-WAR YEARS
In the post-war years, as was the case with other abandoned tunnels throughout Australia, edible mushrooms were cultivated. In the mid 1950s, a mushroom growing business was established by Tom Paskin. He installed a little loco train on tracks and grew his mushrooms in boxes; he supplied these mushrooms to Edgells for canning.
The current exotic mushroom operations, established by Dr. Arrold in 1987, are unique in Australia. The environmental conditions in the tunnel, namely low temperature and high humidity, are ideal for growing half a dozen varieties of exotic mushrooms. The particular environment in the tunnel is very close to the atmospheric conditions experienced in the mountainous regions of China, Japan and Korea where these types of mushrooms grow naturally.
The home-grown Asian varieties such as shiitakes, oysters, wood ears and shimajiis are sold to a niche market mainly in Sydney and interstate.
For those interested in viewing exotic mushrooms growing, a series of tunnel tours are arranged throughout the year.
Information for this article was sourced by Philip Morton from the archives of Berrima District Historical & Family History Society and from the writings of John McColgan, Mittagong historian.