High-speed rail in Australia has been under investigation since the early 1980s. Every Federal Government since this time has investigated the feasibility of constructing high-speed rail with speeds above 200 km/h, but to date nothing has ever gone beyond the detailed planning stage. The most commonly suggested route is between Australia's two largest cities, Sydney and Melbourne, which is the world's second- or third-busiest air corridor. Various corridors have been proposed for a potential high-speed line. The distance between them is around 800 km (500 miles), which requires very high speeds to make trains competitive with air travel.