As 60,000 pounds of steel and fiber-reinforced polymer was slowly nudged toward the deep purple support posts, the small crowd of onlookers chattered excitedly. The long-awaited “canopy-walk” bridge spanning the Mattapan trolley tracks was finally in place, the latest piece of the 1.3-mile Neponset River Greenway extension to become a reality.

Department of Conservation and Recreation workers installed the span on Saturday morning and construction of its canopy-walk is now underway on both sides of the 18-foot-high bridge, which connects the main trail from one side of the trolley tracks and the Mattapan MBTA bus stop to the other.

The elaborate riverside boardwalk will connect Blue Hill Avenue in Boston to Central Avenue in Milton. It is considered the “missing link” in the Neponset River Greenway trail between Mattapan Square and Pope John Paul Park II in Neponset.

Project manager Stella Lensing said the project is over budget but within contingencies. The original budget was just under $14 million, with current projections at just over $14 million, Lensing said.

“For a trail of this length, it is one of our more expensive trails,” she said. “But we have two bridges. There’s a lot of steel on the project.”

Sometime in mid-May, Lensing said, the second span, the Harvest River Bridge, will be set in place by cranes working each side of the river. DCR engineers say the dramatic arched bridge, constructed with Ipe timber – a dense rainforest wood – decking, will involve considerably less coordination because it will not touch on the active transit system.

Lensing expects the expansion will be completed in the fall, most likely in October.

The relatively mild winter allowed Greenway construction to continue later than expected. Most of the subgrade – under the pathway – and support structure work has been completed, with asphalt paving still to cover the existing gravel.

Visually, the unfinished canopy-walk and trail still look a ways away from offering a path fit for pedestrians and bicyclists (Note to hoverboarders and electric bicyclists: No recreational electric/ motorized vehicles are allowed on DCR trails).

“There’s a lot already done, but there’s the final work,” Lensing said. Planting along the walkway will happen between now and May, she said, with other work to be done on cleaning up the riverbank.

Members of Mattapan Food and Fitness and the Neponset River Greenway council and state Rep. Dan Cullinane were on site Saturday morning to see the bridge being installed. The crowd included at various times B-3 police commander Captain Haseeb Hosein, former Neponset River Greenway DCR project manager Kathy Garnett, and Savin Hill’s Paul Nutting, who is running for Suffolk County Register of Deeds.

One local resident in the gathering, Vivian Ortiz, said she is “probably one of the newest people to move to Mattapan, but probably one of the most excited.” She moved to Leahaven Road, right next to the Greenway path, about four years ago.

“I choose public transportation for everything,” said Ortiz, a regular participant with Mattapan Food and Fitness and the Neponset River Greenway Council. She and three other women who stood at the site from 7 a.m. are grant-funded envoys with Let’s Get Healthy, Boston!, which encourages community members to eat better and get involved with active transit.

“A lot of the effort of what this group is doing is just engaging this community to be ready,” Ortiz said, “so that when it does open, we’re out here riding our bikes, we’re out here walking, we’re in active walking groups. So they’ll be able to see that the investment went to the correct community.”