Denver’s Regional Transportation District says it will need about $2.4 billion more than expected to build all components of the city’s FasTracks rail expansion.

The agency, which released its 2010 FasTracks budget in January, puts the total at $6.6 billion, well beyond the $4.2 billion it can collect from taxes in the near future.

RTD oversees bus and rail transit for the six-county Denver metro area. In 2004, the agency spearheaded an election in which voters approved $3.95 billion in new taxes for FasTracks. The program aims to build 122 miles of new commuter-rail and light-rail lines, 18 miles of bus rapid transit and other improvements. It is scheduled for completion in 2017.

FasTracks has faced budget blues since its inception. Higher construction costs, declining tax revenues and sobering revenue projections have forced the agency to reconsider its options. The $4.2 billion available includes federal funds, tax revenues and additional sources but is not enough for the agency to build the FasTracks system as planned by 2017.

RTD is considering asking voters to double the 0.4% tax they approved six years ago. “We don’t know if we’re going to do that yet,” says Pauletta Tonilas, FasTracks spokeswoman. “We’re doing some private polling and focus groups to test the mood out there, and the board will decide in late March whether to go for a vote.”

Tonilas says other options are not on the table for now. “We are not considering any major changes to the program. We will build all of the corridors as promised,” she says. But some might have only single-track instead of double-track alignment.