It's a tiny line on a map that appears to hint at something bigger: the possibility of a third underground rail through the city by 2051.

Maps of Melbourne's future rail network released by the Andrews government showing the $50 billion suburban rail loop, include a familiar, but not-yet-existent rail line connecting the CBD and Newport.

It appears to represent Melbourne Metro 2 - an underground rail line linking Clifton Hill with Newport, via the CBD and Fishermans Bend - and viewed as a hint that, by 2051, the government expects the rail line would be built.

While the future status of Melbourne Metro 2 has not been confirmed, Premier Daniel Andrews was careful not to rule it out when asked about it last week.