The Transbay Transit Center project is now facing a $300 million shortfall, which, in case you aren't sure, is kind of a big number. The first phase of the project, which includes the construction of the five-story transit hub with its rooftop park, has come in over budget in bids from contractors, and in general construction costs are on the rise. This has led to a design change in the cool, undulating building designed by Pelli Clarke Pelli, in which a curved glass skin has been replaced with a perforated aluminum one.

Also, federal security requirements, basically to help ward off terrorist attacks, added an unexpected $50 million to the budget. In total, the first phase is now likely to cost $1.9 billion, rather than the $1.6 billion originally estimated.

The Transbay Joint Powers Authority, which is responsible for building the complex, is shifting money around to cover the shortfall, including $50 million in toll bridge revenue that was meant to cover future expenses. The second phase of the project, which involves connecting the rail lines at Fourth and King Streets to the transit hub at First and Mission, is not currently funded and money budgeted for that phase is being shifted to the first phase as well. This could, potentially, jeopardize future federal funding for the project, as the CalTrain link is key to competing for those funds.

Supervisor Scott Wiener, who serves on the Metropolitan Transportation Commission, called the new budget revelations "disappointing," but assures the Chronicle, "We'll get it done."

Final approvals for the project go before Planning in October, and the construction of the above-ground superstructure is scheduled to begin next spring, with the whole thing completed in late 2017.