A larger, 4/5-mile section of the viaduct stretches with fortresslike walls from Fairmount Avenue to Vine Street. Across Broad, the old railroad line drops below street level, running through a subterranean channel from the former Inquirer and Daily News building to Fairmount Park at Girard Avenue.
But, for now, the focus is on the spur - which has the advantage of being owned by SEPTA, a cooperative public agency.
The rest of the elevated viaduct remains in the hands of the old Reading Co., a 19th-century railroad power now transformed into a California real estate venture that owns movie theaters. The city had conversations with the company several years ago, but nothing since.
"Before we go to them about the rest of the viaduct, let's demonstrate that we can take that first step," said Paul Levy, chief executive of the Center City District. "We're looking at this as sort of a proof of concept."
Work on the spur, which would include landscaping and adding stairs to street level, plus some environmental remediation and waterproofing of the leaky underside, is expected to cost $8.6 million.
The city has pledged $1.8 million over two years from its capital budget, while the state has included $3.5 million in its Redevelopment Assistance Capital Program.
Neither sum is guaranteed, and private money will be necessary in any case. Levy said the William Penn Foundation, which supported a viaduct feasibility study in 2010, has pledged funding for construction but isn't ready to reveal how much.
The project that was the inspiration for the viaduct - New York's High Line, built on an old, elevated freight line - helped spark a building boom on Manhattan's once-desolate West Side.
Levy and the Center City District, which serves as a business improvement mechanism for the downtown area, became interested in the viaduct after seeing the High Line's success.
The viaduct slices through an area between Chinatown and Fairmount Avenue that has been growing through loft conversions and other new housing, but remains something of a wasteland of former factories and warehouses. Levy has referred to the neighborhood as the "hole in the doughnut" of Center City's continued expansion.
The neighborhood activists, though, aren't waiting for government dollars - they already have begun planning for the caring and programming at the rail park. (Although the property would come under control of the city Department of Parks and Recreation, some sort of partnership with a neighborhood nonprofit is likely.)
Sarah McEneaney, an artist, and John Struble, a furniture-maker, teamed up in 2003 to found the Reading Viaduct Project. Last fall, they merged that organization with Friends of the Rail Park, a group dedicated to transforming the underground section west of Broad.
The two groups joined forces after working together on a block party to raise money for maintenance of the viaduct park. So far, they have raised more than $70,000.
"We started out as advocates," McEneaney said. "We know we're going to morph into caretakers."
The once-mighty Reading Railroad had been a victim of changing times. The viaduct last carried trains in 1984, when the Center City commuter rail tunnel opened, linking Reading's old routes into SEPTA.
But the railroad's name and legacy have lived on here, and not just because of its spot on Monopoly boards. Reading's headhouse now serves as the ornate Market Street frontage for the Convention Center, and the train shed is now better known for the Reading Terminal Market.
If the rail park is completed, the project would be another example of Reading's antiquated infrastructure being folded elegantly into the city's modern life.
Deputy Mayor Alan Greenberger, who has supported the viaduct project, said completing the spur should create momentum.
"It will turn out to be a great little park," he said. "If nothing else, it will make people clamor for more."