The sleek, computer-driven architectural previews of New York’s first midair park, the High Line, depict pedestrians navigating a public promenade that is on track to be celebrated next fall. Like space-age schematics, they offer a futuristic refuge: a pristine ribbon of green providing exquisite views of Manhattan.

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Damon Winter/The New York Times

Construction workers have been toiling on the elevated rail platforms so that part of the High Line park can open next fall. More Photos »

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Working on the High Line

But the High Line has been something quite different, a flaking, rusting industrial ruin that needed to be transformed to match the digital renderings. And someone has been doing all that work. So right now the High Line is one hairy construction site.

The defunct elevated railway — which stretches from Gansevoort Street to 34th Street on the Far West Side — is a secret world these days, barred to the meatpacking district crowds that mob the new Apple Store and swarm to a high-end shopping festival in a once-scruffy quarter that real estate advertisements now call “the prestigious High Line District.”

Fifty hard hats in safety orange — including ironworkers, carpenters, painters and garden-variety laborers — perform a fast-tracked logistical ballet 30 feet up on the line, as steel and concrete are delivered just in time to be grappled into place.

Bridges freeze before roadways, of course, and thus it is on the High Line, which shimmers with icicles at times while vibrating with hard winds from the Hudson. Safety railings sing in the gales, and it is not uncommon for snow and sleet to blow upward, swirling in updrafts shaped by the patchwork of low-rise buildings underneath.

Not unlike the hardy wildflowers, shrubs and even apple trees that adapted to the lost world of the track bed, workers have already embraced the onset of winter.

“The cold is great — bring it on,” said John Forbes, 43, an ironworker. “We really don’t mind cold. It’s heat that’s the killer.” He referred to the summer’s labor on the unshaded railway radiating hotly from its 8-inch-thick concrete slab.

Of course, high wind — as on a recent afternoon punctuated with chilly gusts of 40 miles an hour — forces managers to shut down the construction cranes. A freeze curtails the concrete pours and painting forays, while ice and snow divert topside workers to their shovels before they can scurry to tasks on the line’s dry undersurface.

The project that has been promoted as the new Central Park for downtown is, currently, a mile-long obstacle course. The rail bed threads its way not only through High Line construction but also 10 other developments, including a new tunnel through the Standard Hotel at Washington and Little West 12th Streets.

“It’s very, very tight up here,” said Mike Balbo, 27, back on the High Line. He was behind the wheel of a 9-ton payloader ferrying job debris. “Just fitting this on the road is hard.”

Bob Marriott, general superintendent for Kiska Construction Corporation, the general contractor, said, “We’ve been trying to complete our repairs and our painting without massively disrupting the businesses and tenants below.”

Save for a parcel near Gansevoort Street, the city owns none of the real estate underneath the High Line aside from streets and sidewalks. “You do not want to drop things from on high,” said Gerard Zimmermann, 40, a chief inspector for Kiska.

The roadbed’s elevation is nothing, of course, to workers accustomed to dancing on high steel. “For me this is pretty easy,” said Mr. Zimmermann, who has walked atop the George Washington and Verrazano-Narrows Bridges.

Nevertheless, the airborne landscape poses safety issues and other, more personal constraints. For example, since sanitation contractors do not deign to scale 30-foot heights, the workers must descend from the line “because companies will only clean portable bathrooms downstairs on the street level,” said Garrett Scalza, 30, who was supervising a group of carpenters near Gansevoort Street.

And since the High Line extends through residential areas, “We can’t make noise early or late, or work on the weekends,” Mr. Zimmermann said.