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Railway line in Sydney, New South Wales, Australia

For most of its length, the line travels underground. It runs south from platform 23 at Central station across a viaduct to the tunnel portal beneath Prince Alfred Park near Chalmers Street. The tunnel roughly follows George Street underneath the suburbs of Redfern and Waterloo. At Green Square station, beneath the intersection of Botany Road, Bourke Road and O'Riordan Street, the line continues beneath Bourke Road to Mascot station, a block south of Gardeners Road. From Mascot, the line roughly follows O'Riordan Street before turning sharply to the west once underneath Sydney Airport. The line runs westward under the Domestic and International terminals before continuing north west underneath the Cooks River to reach the surface at Wolli Creek where it joins the East Hills line. The line is two tracks for its entire length. The two new stations which were built for the airport's International and Domestic Terminals, feature larger lifts and wider ticket barriers to cater for passengers with baggage. Three new suburban stations were built – one each for the residential development areas of Mascot and Green Square, and an interchange station with the Illawarra line at Wolli Creek.

In 1990, the State Government called for Expressions of Interest to build a line to the airport. In July 1994, the Government announced it had entered a public private partnership with Australian investment company Transfield Services and French construction company Bouygues to build the line. Under the deal, a private company, Airport Link Company, would cover the costs of building four of the stations. In return it would operate those stations for 30 years and have the right to impose a surcharge on fares for their use. The company's involvement was predicated on passenger estimates and train reliability guarantees that later proved to be optimistic. The State Government would fund (and own) the railway itself and Wolli Creek station. At the time, the main public transport link between the city and its airport were two Sydney Buses express routes, the 300 to Circular Quay and 350 to Kings Cross branded as Airport Express. A tunnel boring machine was used for the construction. Manufactured by the German firm, Herrenknecht, it arrived in Australia in October 1996. While the use of a Tunnel boring machine relieved the need for large numbers of workers at increased pressure, a caisson system is formed at the cutting head. Workers entering this space for inspection, maintenance and repair had to be trained. As the stations commenced operation on 21 May 2000, the ownership of the four stations reverts to the state government on 21 May 2030. Despite the cancellation of the rival Airport Express bus service, taxi surcharges and expensive airport parking, the Airport Link consistently failed to meet patronage targets. Less than a year after the line opened, the State Rail Authority stated that "patronage has been lower than expected to date", but they remained optimistic, believing "that as airport users become more familiar with this facility and the ingrained habits of many years gradually alter, patronage will continue to increase." State Rail blamed "lower than expected patronage" and stated it was working with the company to increase it. Together with the Cross City Tunnel, the Airport Link served to dampen government and business enthusiasm for further public private partnerships in transport in New South Wales. In October 2005, the Government and the company signed a revised agreement on revenue and patronage, settling the latter's claims against the former. The Government paid to the company, with another due as CityRail earns revenue from Airport Line business. The Airport Link Company was put up for sale in early 2006, and was purchased by Westpac. In 2009, the business made a profit of which increased to in 2010. In 2013, Westpac's 49.9% stake was purchased by Universities Superannuation Scheme (USS), who then took control of the Airport Link Company.

From opening, all stations on the line charged a station access fee to use the line, as well as the standard fare. In March 2011, it was announced that the NSW Government would cover the cost of the station access fee at Green Square and Mascot stations, meaning that passengers no longer need to pay a surcharge to access these stations. To compensate Airport Link Company, the Government pays the company a "shadow" station usage fee at a fixed contracted rate of approximately per entry and exit of these stations. A fee remains in place for the Domestic and International Airport stations, which has been criticised. Workers at the airport do not receive a discount on the station access fee, merely a $21 weekly cap in the additional Station Access Fee. By the late 2010s, Green Square and Mascot stations were experiencing overcrowding at peak times due to significant increases in patronage. At Mascot station, passenger numbers increased by 117% between 2015 and 2019, and the station is now being expanded to deal with the increased demand. The 30 year public–private partnership will end on 20 May 2030, at which point the infrastructure will belong to the NSW Government and the station access fee will be removed.