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After drawing rave reviews for original plans by renowned architect Santiago Calatrava, the planned World Trade Center transit hub has undergone considerable changes.

The initial plans called for a mobile spindly glass roof, which represented a bird in flight. But security concerns forced the elimination of glass from the winged roof, which was also shortened. Then, just two months ago it was decided the roof would no longer open and close, due in part to cost overruns.

"It's a number of issues. I mean, yes there's an issue of resources and making sure that it's built as cost-effectively as possible. Secondly, there's the issue of having it fit with the rest of the projects on the site," said Port Authority spokesman Steve Sigmund.

The WTC projects have tight overlaps. Since part of the memorial plaza will sit atop the station mezzanine’s roof, the mezzanine must be completed soon to reach the memorial’s completion deadline of 2011. Meanwhile, the aboveground structure sits so close to the nearby towers, that an open roof would have left just six feet of clearance.

"You have to design a mezzanine under the memorial, part of the transportation hub, that can support that memorial. And you have to design a wing structure on top that can fit within [WTC] Towers 2 and 3," said Sigmund.

There may be further changes ahead as the Port Authority struggles to keep the project within its $2.5 billion budget. The agency is considering, for instance, whether to eliminate plans for a mezzanine designed to be free of columns.

Many in the downtown community say the transit hub, if done right, will help attract tourists to the site.

"They will come to the memorial and they'll come to the museum, but they're also going to look at the grand architecture, and how we decide as a community to rebuild and commemorate that tragic day,” said Community Board 1 co-chair Catherine McVay-Hughes.

The Port Authority says the transit hub will preserve as much of Calatrava's original vision as possible. The transit hub, originally scheduled for completion next year, is still several years away from completion.

PATH trains have been running out of the WTC area since 2003 and will continue to run throughout the construction process. A temporary PATH station entrance on Vesey Street opened in March, replacing the old one on Church Street, and will probably continue to be the main station entrance until work on the hub is complete.