Geotechnical drilling for the airport rail link has finally begun, as the Andrews government pushes ahead with a rail line to Melbourne Airport - a project that has been debated for six decades.

In the first sign of works on the long-awaited airport train, the government has launched 12 weeks of soil testing across six sites along the proposed route between the city and the airport via Sunshine.

Credit:Chris Hopkins

The testing around the M80 Ring Road in Tullamarine and along the Maribyrnong River will gather information about ground conditions and soil quality, which will help inform the full business case to be finalised either this year or in 2020.

But as the government started work on the $300 million full business case, it refused to release the preliminary business case it completed last year, which would provide evidence for the choice of route.