PHILADELPHIA—Theresa Lounsbery sets out from her apartment in this city’s blue-collar Lower Northeast section each workday. Her neighborhood, Oxford Circle, a district of 2 square miles, has been hammered by manufacturing’s long slide. Median household income in her immediate area is down more than 30% since 2000 to roughly $37,000.

But less than 40 minutes later, the 43-year-old sat on an elevated train approaching Philadelphia’s Center City, a 7.7-square-mile mini-boomtown framed by a skyline about to reach new heights with...