to provide new underground stations. In 2015 the State Government established the Melbourne Metro Rail Authority to oversee planning for new twin 9-kilometre rail tunnels through the central city between South Kensington station and South Yarra. The new Metro Tunnel will have five new underground stations and connect the Pakenham and Cranbourne lines with the Sunbury line, creating a new route through the CBD as an alternative to the City Loop. New underground stations will be created at Arden, Parkville, Domain and two new CBD stations, State Library and Town Hall. Enabling works were underway by 2016 and major construction work on the tunnel and stations began in 2017. The project has an estimated cost of $11 billion and is scheduled to be complete by 2025. Airport rail link A rail link to Melbourne Airport has been proposed repeatedly since the airport's construction, with a variety of routes and service models suggested, but construction has never commenced. In July 2018, the Federal and State Governments each pledged A$5 billion (for a total of A$10 billion) to construct a rail link. The Federal Government proposed four preferred routes for the link, with one proposal running via a direct tunnel to Highpoint Shopping Centre and the others linking to existing stations in Broadmeadows, Flemington or Sunshine. An assessment of the four preferred routes conducted by Rail Projects Victoria culminated in the route via Sunshine station being selected, with a detailed business case expected by 2020. Construction began in 2022 and the line is set to open by 2029. Suburban Rail Loop In August 2018, in the run-up to the 2018 Victorian state election, the State Government pledged $300 million to complete a business case and secure funding to construct a new railway through suburban Melbourne. The project is designed to link major activity centres and amenities such as hospitals, shopping centres, universities, and Melbourne Airport. The proposal would connect most existing railway lines through middle suburbs and enable easier intra-suburban travel. The Suburban Rail Loop (SRL) would connect the existing station at Cheltenham with other existing stations at Clayton, Glen Waverley, Box Hill, Heidelberg, Reservoir, Fawkner, Broadmeadows, Sunshine and Werribee. It will also link to new stations to be built in areas that have long been promised rail connections, including Monash University, Burwood, Doncaster, Bundoora and Melbourne Airport. Construction on the first stage, SRL East, began in 2022 and is set to