Plans for the Melbourne Metro rail tunnel have been dusted off and relaunched, continuing years of parliamentary chopping and changing of the state's largest rail project since the City Loop.

Immediate funding of $40 million has been made available by the Andrews government to begin planning the twin rail tunnel through Melbourne's CBD with five new stations, a project first announced by the Brumby government then heavily altered under former premier Denis Napthine.

But Melbourne Metro ultimately remains dependent on billions of dollars in federal funding, with the Andrews government yet to secure any Canberra cash from Prime Minister Tony Abbott, who rejects a federal role in funding urban rail.

Nevertheless, on Monday a tentative opening date of 2026 was given for the project, with construction expected to begin in 2018.