With both the Victorian Government and State Opposition offering competing visions for an expansion of Melbourne's underground rail network, it is useful to get a sense of the scale of what is proposed.

Minister for Transport the Hon Vernon Wilcox QC excavates the first soil in the construction of the Melbourne Underground Rail Loop on 22 June 1971 ( Supplied: Public Record Office Victoria )

Thousands of photos held by Public Record Office Victoria (PROV) document the construction of the City Loop, Melbourne's last great underground rail project.

These photos, previously unseen by the general public, also highlight how much Melbourne has changed as a city and as a society.

They show smoking workers, out-dated technology, retro fashions and reveal a city skyline largely untouched by skyscrapers.

Digging the pilot tunnel for the Melbourne Underground Rail Loop on 10 August 1972. This is one of a number photos of the City Loop works held by Public Record Office Victoria that show workers smoking, a sign of how much society has changed since the seventies. ( Supplied: Public Record Office Victoria )

The office staff keeping the project on track at the Melbourne Underground Rail Loop's Queen St headquarters, photographed 5 August 1971. ( Supplied: Public Record Office Victoria )

The images shown on this page are a small sample of the images kept in PROV's climate controlled vault at its North Melbourne facility.

Jackhammering the face of the pilot tunnel for the Melbourne Underground Rail Loop in Adderley St, North Melbourne on 8 June 1972 ( Supplied: Public Record Office Victoria )

Also kept in the archives are documents from the planning phase of the project.

These include a hand-written spreadsheet of every survey carried out in the construction area since the 1800s.

Also on file are engineer's drawings of the different types of digging machinery which could be used and a categorised bibliography of publications relevant to the project.

It is, however, the eleven boxes of photos detailing every step of the City Loop's construction that impress the most.

Construction of the Melbourne Underground Rail Loop's Jolimont access tunnel, as viewed from the Reserve Bank building on 6 October 1972. Note the old Melbourne Cricket Ground in the background. ( Supplied: Public Record Office Victoria )

Steel framework for the Jolimont access tunnel of the Melbourne Underground Rail Loop, taken on 8 November 1973. The Jolimont portal was dug using the cut and cover technique, rather than the mining techniques used for Flagstaff and Parliament stations. ( Supplied: Public Record Office Victoria )

A man works on the steel structure for an entrance to Melbourne's City Loop on 24 January 1974. ( Supplied: Public Records Office Victoria )

The first sod was turned on what was then known as the Melbourne Underground Rail Loop on 22 June 1971.

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In a nod to the scale of the project then, Transport Minister the Hon Vernon Wilcox QC dug the soil in an excavator.

An ABC TV report aired on New Years' Day 1974 outlined how "below the city streets construction workers using the latest tunnelling techniques and sophisticated machinery work around the clock."

According to the report the first trains were expected to run through the loop in 1978.

However, as with many large projects, delays crept in and the City Loop officially opened on 24 January 1981.

Even then Museum was the only underground station completed, with Parliament and Flagstaff stations not opening until 1983 and 1985 respectively.

Construction of Flagstaff Station on 23 April 1974. While works look well advanced, Flagstaff would not open to passengers for more than a decade. ( Supplied: Public Record Office Victoria )

Building the 'filler wall' at the south end of Parliament Station, 30 June 1975. ( Supplied: Public Record Office Victoria )

Flagstaff Station was more difficult to construct than Parliament Station due to the geological make-up of the area. This photo was taken 28 April 1978. ( Supplied: Public Record Office Victoria )

Drill sampling at the Adderley St tunnel site in North Melbourne, 10 July 1975. ( Supplied: Public Record Office Victoria )

An excavator works to clear part of the Adderley St rail tunnel in Noth Melbourne on 15 May 1978. ( Supplied: Public Record Office Victoria )

Coop's shot tower, built in 1888, is now enclosed by Melbourne Central shopping centre. This photo was taken in the 1970s, though the exact date is not known. ( Supplied: Public Record Office Victoria )

Different techniques were used to construct different parts of the loop.

The "cut and cover" technique was used to build Museum Station (eventually known as Melbourne Central) and the entry portals at Jolimont and North Melbourne.

Flagstaff and Parliament stations were dug and built using mining methods.

Flagstaff Station in particular was a challenge because of the complex geological make-up of the site.

The technical solution to this challenge was considered remarkable enough to be included in a book published by the Australian Academy of Technological Sciences and Engineering for Australia's bicentenary.

Construction of Museum Station required that La Trobe St be re-routed around the work site.

Both the road and the tram tracks were redirected from late 1973 until 1978.

Telephone lines also had to be diverted to make way for the trains.

Construction in the Jolimont rail yard on 28 May 1973, looking back towards the city. Melbourne's skyline has changed significantly in the four decades since this photo was taken. ( Supplied: Public Record Office Victoria )

La Trobe St and its tram tracks were re-routed during construction of Museum Station, now known as Melbourne Central Station. This photo was taken on 1 October 1975. ( Supplied: Public Record Office Victoria )

Diverting telephone lines to allow for the City Loop tunnel works, 28 September 1977. ( Supplied: Public Record Office Victoria )

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Two hundred people took the first trip around the loop on 6 December 1981.

An ABC TV News report noted that most of the people on the journey were VicRail staff.

In the report Transport Minister Robert Mclellan said the first trip around the loop had an "emotional significance".

"It's working, it's now going to be used by the public," he said.

A truck converted for use as a rail vehicle in a tunnel at Museum Station, 22 August 1979. ( Supplied: Public Record Office Victoria )

Preparing the platforms at Flagstaff Station, 11 November 1982. This is another photo showing workers smoking. ( Supplied: Public Record Office Victoria )

At the time of opening it was claimed Parliament Station had the longest escalators in the southern hemisphere. This photo is marked as 15 June 1983; however, the station reportedly opened on January 22 that year. ( Supplied: Public Record Office Victoria )

Thanks to the staff at ABC Archives and Public Record Office Victoria who made this feature possible.