The Queensland government will commit another $2 billion to Brisbane's Cross River Rail, after failing to secure federal funding for the project ahead of the budget.

The money, understood to be allocated in the state budget on Tuesday, takes the Palaszczuk government's spending to roughly $2.85 billion, leaving a significant gap to get the project off the ground.

The state describes the project as "critical" for keeping the south-east corner's train network flowing ahead of an expected bottleneck on the city's only north-south rail bridge across the Brisbane River.

The Commonwealth government has chipped in $10 million towards planning, tied to integration with the Brisbane City Council's proposed Brisbane Metro project, but refused to commit to more.