“The BeltLine doesn’t go where people want or need to go,” said Michael Dobbins, an architecture professor at the Georgia Institute of Technology, who has studied the project’s feasibility. “The parks and trails are great, but it makes no sense to add streetcars while traffic elsewhere is so bad, especially in this economy.”

But supporters point to signs of progress: 60 acres of parks have been built and five miles paved for bike baths in the past five years. Thousands of people walk and bike along the Eastside Trail, which runs from the city’s largest park to the historically black and rapidly gentrifying Old Fourth Ward neighborhood, where the Rev. Dr. Martin Luther King, Jr., was born and lived.

Mr. Reed said rising property values, private donations and federal grants will bridge the financial gap. “These changes are happening in the teeth of the worst economy in 80 years,” he said. “The pace of the BeltLine will pick up.”

“Rails-to-trails” projects are gaining steam across the country. Every year, 450 miles of railroad fall out of use. Cities are converting the unused tracks into green space and bike trails.

In Chicago, an elevated three-mile path is being built atop an old freight line. Seattle is turning 13 miles of track into bike trails. Four million people a year travel on New York’s version, the High Line, which runs along an elevated platform above Manhattan.

In total, more than 21,000 miles of railroad track has become paths across the country, according to the Rails-to-Trails Conservancy. The nonprofit group says 9,000 more miles are available to be converted.