A high-speed train line from the CBD through Melbourne's western suburbs is the best way to deliver a "fast, frequent and affordable" airport rail link, the airport's operator says.

With impetus growing for the long-awaited rail link, the Australian Pacific Airports Corporation has for the first time gone public with its preferred route, saying a line from Southern Cross to the airport with a major interchange at Sunshine is the “clear front-running route” for the project.

Airport executive Lorie Argus Credit:Chris Hopkins

The company says it is enthusiastically advocating for a train line, which might cost up to $10 billion, as a much-needed alternative transport option for the 35 million air passengers who go through Tullamarine each year.

Prime Minister Malcolm Turnbull, who has pledged up to $5 billion in Commonwealth money to help build the line, and Premier Daniel Andrews met in Melbourne this month to discuss the project.