Most public space projects in Philadelphia tend to stew for years, so let's give the Center City District credit for placing its redesign of City Hall's plaza on a fast track. The drawings started making the rounds of Philadelphia's various design commissions earlier this year. Now, the CCD has released its "final" design for Dilworth Plaza.

Let's hope they're using that term loosely.

There's certainly much to admire in the plaza's renderings, which show a simple lawn, a flat multipurpose area, a cafe, and two curvy transit headhouses, wrapped in a cottony pouf of trees. But peek below the surface, specifically to the underground transit area, and this $45 million design by KieranTimberlake and Olin still has some miles to go.

The CCD's vision for Dilworth Plaza, completed only in 1977, is really two projects in one. It's an effort to create a worthy public space in Philadelphia's civic heart, one that is more park and less plaza. Yet, it's also a major transit project intended to provide a gleaming gateway station for the city's underappreciated underground rail network. The problem is that the transit portion is dictating the design of the public space in ways that are good for neither.

That's not to diminish the need for better transit stations. Both the plaza and SEPTA's City Hall station are in a disgraceful state. What should concern city and SEPTA officials is the grandiosity of the CCD's proposed transit room, which would span nearly two blocks below the plaza.

Let's remember that there is already a warren of underground transit facilities in the two blocks west of City Hall, ranging from the Broad Street subway station to the Penn Center concourses to Suburban Station's regional rail platforms. Is another big transit space below Dilworth Plaza really going to make this mess cohere?

Admittedly, a dank, winding corridor already exists under Dilworth Plaza, linking the city's Municipal Services Building to South Broad Street. It was created by Philadelphia's late master builder and planner Edmund Bacon as part of a then-innovative scheme to integrate transit, government services and the workplace.

Bacon imagined Philadelphians streaming into Center City by rail to pay taxes and obtain permits at the MSB's underground counters. Without stepping above ground or crossing a city street, people could follow the concourse to shops or a job in one of the interconnected Market Street office towers.

If the scheme was a bit utopian then, it's definitely outmoded today, since people increasingly pay bills and access city services online. We also understand now that people prefer to go about their business up on the sidewalks, in the light of day.

The CCD's Paul Levy recognizes that the concourse needs help. His organization, which is funded by downtown businesses, recently took over maintenance and has greatly improved its cleanliness. That experience is partly what got his private group interested in redesigning Dilworth Plaza.

The area, both above and below, is unnecessarily complex, with cumbersome changes in levels and inexplicable twists and turns.

Levy's initial concept was to simplify: Smooth things out on the plaza. Create a straight corridor below from north to south. Olin's simple, almost minimalist, plaza design acts as a welcome low-fat side dish to City Hall's ultra-rich facade. So far, so good.

But the transit portion has taken on a life of its own. Instead of just straightening out the underground passage, Levy and the designers now propose a large "transit room" - a full-size train station, really - that would provide a centralized entry for underground rail. For the first time, entrances to the city's two subway lines would face each other in one space.

But the proposed room is so vast that Levy is already talking about installing graphic panels to liven things up, perhaps with historical details about the construction of City Hall. "We think of them as one more reason to come down and take transit," explained Levy, the CCD's president.

Hmmm. Anytime history panels are needed to make an architectural space interesting, alarm bells ought to ring.

They're a sign that the transit room is overscaled. Subway riders don't linger in a station in the same way as suburban commuters, whose trains may depart only once an hour. It's also worth noting that more than 75 percent of riders now access underground rail from the area west of 15th Street. That's where the Market Street office towers are.

This is important because the centerpiece of the redesigned plaza would be two swooping glass headhouses leading to the proposed transit room. Designed by architect Richard Maimon of KieranTimberlake, they are very elegant structures, suggesting in their curves a giant circular frame for City Hall. But does Philadelphia need them?

I suspect that transit dominates the two-prong design because that's where the money is. Since the plan would provide SEPTA's subways with much-needed elevator access, Levy hopes to tap into federal transportation funds for at least a third of the $45 million cost, with the state and the CCD picking up the rest.

There may well be a case for making Dilworth Plaza the city's central train station. You can argue, as Levy has, that the city's subways should be more welcoming to tourists, who will be spending more time on Broad Street once the Convention Center shifts its main entrance there.

But to be convincing, you can't look at the plaza in isolation. The city needs to understand the role played by each of the three squares in this municipal plaza-land, as well as the plaza space in front of the Penn Center and Centre Square towers.

Every one of these spaces has a transit entrance. Perhaps Dilworth Plaza would be better off with different structures. A real restaurant pavilion would be nice, not just a cafe.

The city is belatedly forming a committee to look at the bigger picture. Meanwhile, the Dilworth Plaza redesign keeps chugging along, collecting city permits. Next up, the Art Commission on July 1.

Contact architecture critic Inga Saffron at 215-854-2213 or isaffron@phillynews.com.