It's easy to imagine Philadelphia's business district accommodating another tower the size of Comcast. After all, the region's entire transit system funnels downtown, making the city's core accessible to thousands of commuters. Now, try to visualize the same amount of office space — 1.2 million square feet — in the neighborhood tucked below the South Street Bridge, a tight enclave of rowhouses and corner pubs once known as the Devil's Pocket.

Assuming America's health-care industry keeps growing at its current rapid clip, the area around the Pocket could soon see a development with even more square footage than the Comcast Center. Children's Hospital of Philadelphia is wrapping up an ambitious master plan that would replace a gouged-out factory on the city's emerging Schuylkill waterfront with a dense cluster of modern research buildings, green parks, a bike trail, a 1,600-car parking garage, and a smattering of restaurants and service retail.

The proposed medical research campus is an encouraging confirmation of Philadelphia's potential as a center of innovation. The health-care industry and higher education are now the city's dominant industries, the linchpin of its economy. Expanding to the east bank of the Schuylkill would give the West Philadelphia Children's Hospital a prominent foothold near Center City, provide research facilities to enhance its reputation nationally, and bring thousands of new, high-paying jobs to Philadelphia — all within a short walk of its main campus.

At the same time, a project this immense can't help but leave a visible imprint on the adjacent neighborhood, a booming area where hundreds of middle-class homes have been built in the last decade, including the high-end Naval Square development. Density is nearly always a good thing in cities, but not all areas can handle the same amount of density. Although SEPTA's University City station is a quick walk over the South Street Bridge, the Pocket, and the larger Grays Ferry section, are not well served by transit. And, as the name suggests, the Pocket is tricky to reach by car. The research center could be a boon to the neighborhood — or it could crush the life out of it. The devil in Devil's Pocket will be in the details.

The hospital is clearly aware of the potential impact of its immense medical campus, especially from car traffic. It has hired Cooper Robertson & Partners, the same planning firm that oversaw master plans for the Delaware waterfront and New York's Battery Park development, to devise a strategy to cushion the shock of the expansion, which would be executed in phases over 15 to 20 years.

Although the hospital's master plan for the nine-acre site — located between the river and Schuylkill Avenue — is still being tweaked and tested, the hospital recently presented the outlines at a community meeting held at Naval Square. At The Inquirer's request, the hospital's president, Madeline Bell; vice president for facilities, Douglas E. Carney; and Cooper Robertson's Donald Clinton agreed to discuss the project in more detail.

One thing is immediately clear: This is not your average medical campus, with aloof towers hovering above fortresslike garage podiums. Cooper Robertson has thoughtfully distributed the 1.5 million square feet of space in a more urban format that is friendlier to Philadelphia's pedestrian scale.

Three towers, of about 26 stories each, would be lined up along Schuylkill Avenue. At least two would have entrances at street level and some public ground-floor uses. Free-standing garages would anchor opposite ends of the long site. The hospital says it would camouflage the parking structures with green roofs and, possibly, ground-floor retail, in the hope of creating a mixed-use, 24/7 environment.

The project also promises benefits for the hugely popular Schuylkill trail. The hospital is in the process of deeding over to the city a 60-foot-wide strip of river frontage, running the entire 700-foot length of the property. Although the city's Schuylkill River Development Corp. would have to raise money to build and landscape the extension, the gift could provide a terrific amenity for the public and for hospital workers. Cooper Robertson has made space for the city to build a public path through the hospital site, linking Schuylkill Avenue to the trail.

All these ideas are excellent, and show a strong sensitivity to the surroundings. James Campbell, a local architect and planner who is part of a neighborhood committee that met with the hospital to discuss the plan, says he was "amazed to see them incorporate many of our ideas" for improving riverfront access.

The real issue is what happens when the thousands of new workers — the hospital is coy about providing an exact number — converge on the Pocket. "We know," Carney acknowledges, "that we have to solve the transit issue." That could involve running private shuttles or persuading SEPTA to increase bus and train service.

The good news is that Children's Hospital's 9,000 employees in West Philadelphia have a long tradition of getting to work without relying on cars. According to officials, an astounding 90 percent walk, bike, or take public transit. The hospital expects the ratio to remain the same at its east-bank campus, which will focus on analytical research and not be required to accommodate patients.

Still, the master plan calls for ultimately building 1,600 spaces. Carney argues the number is low for the size of the proposed campus. Maybe so, but in absolute terms, it's still a lot of people driving into the disconnected Pocket. It will also add two more garages on the riverfront, part of a lineup that includes the Amtrak and IRS garages a few blocks north.

To avoid bringing traffic into the Pocket's narrow streets, the hospital would build a ramp to bring cars from the South Street Bridge directly into the northernmost garage.

The garage would match the height of the bridge, which is roughly 35 feet above the Schuylkill Avenue site. The master plan shows its roof covered by a landscaped park that would serve as a pedestrian gateway to the research center. The park would fan out, leading to an elevated pedestrian esplanade that would cantilever over the CSX rail tracks, offering spectacular views of the river and the hospital's West Philadelphia campus. The esplanade would also serve as the research center's outdoor street, linking the three towers and two garages.

The idea of strolling through the charming streets of Center City, into the research campus' parks and paths, down to a landscaped bike trail sounds idyllic. It's meant to.

The hospital sees the amenities, as much as the research facilities, as part of its sales pitch to help attract topflight researchers, Carney explains. Few health-care campuses can boast such an easy walk to recreation and downtown culture. The parks, esplanade, and bike trail would also be available to the public.

But these amenities, unfortunately, are hugely expensive. Such eye candy is often used to help sell master plans, but is rarely built. This project may boil down to its first phase: a 600-car garage and a single tower. What will this enormous complex be like if the generous public spaces and promised restaurants get cut to contain costs?

There was a similarly ambitious master plan for the medical district along Civic Center Boulevard. It promised manageable urban blocks, lined with wide sidewalks that could accommodate outdoor cafes. Instead, the driveways and porte cocheres are wide and the sidewalks narrow on that car-oriented stretch.

Joseph Syrnick, who heads the riverfront development agency, told me the esplanade "is a fantastic idea whose chance of happening is lower than zero." CSX would never allow a structure to hover over its tracks, he predicts.

As we've seen before, good intentions aren't always enough to ensure that large modern construction fits comfortably into the fine warp and gentle scale of Philadelphia's colonial-era grid. The hospital plan proposes some of the best ideas we've seen in a while for the city's ill-treated waterfronts. City officials should make sure those amenities are realized. Maintaining the livability of great neighborhoods like the Pocket is crucial to Philadelphia's future.