Transport and cities program director at the Grattan Institute Marion Terrill believes the $34.4 billion first stage of Suburban Rail Loop is an idea created in a “very secretive environment”.

The State Government is pushing ahead with tunneling work on the loop, despite no extra funding being pledged from the Albanese government.

“It’s been an idea dreamed up in a very secretive environment,” Ms Terrill told 3AW Breakfast.

“It was launched three months ahead of the (state) election and the government has stuck to it very strongly.”

Click PLAY to hear how the Suburban Rail Loop could now come at the expense of other major projects