In short:

A new report on the business case for Victoria's Suburban Rail Loop has prompted the federal government to request the state government "come forward and explain".

The Infrastructure Australia report casts doubt over the financial viability of Premier Jacinta Allan's signature project.

Victorian government ministers faced intense questioning from reporters on Saturday over the transparency of their costings for the $34.5 billion rail project.

The report by Infrastructure Australia said the advisory body had low confidence in the $34.5 billion cost estimate for stage 1 of the orbital train line to connect Box Hill to Cheltenham, with a potential cost blowout said to pose a "major risk" to the project's viability.

It also said the Victorian government's business case lacked detail and overstated the project's benefits, in part because it assessed the first two stages together when instead they will be delivered up to 18 years apart.