Miami-Dade commissioners voted last week to approve additional funding needed for design work for the Northeast Corridor commuter rail line.
It is the final design work needed before a possible grant approval, according to Brightline.
The new service will include up to five new stations between downtown Miami and Aventura, where it could then continue into Broward County and beyond.
Although it would share tracks with Brightline, it may be separately branded and operated, Brightline said.
The project received National Environmental Policy Act approval from the Federal Transit Administration in November, which allows the project to proceed further.
A memo from Department of Transportation and Public Works director Eulois Cleckley said the additional funding for design work needed to be approved by commissioners in January, so that the project could reach 60% completion of the engineering design by August 2024.
If the project design was not at 60% by then, it could jeopardize prospects for federal and state funding.
The project already has commitments for over half of the estimated $578 million project cost.
The final project funding component is expected to be provided by the Federal Capital Investment Grants New Starts grant program, for which Miami-Dade anticipates receiving a Medium-High overall project rating.