Contractors last week lowered into place the new rail bridge over the Petaluma River, completing a key piece of infrastructure in the Sonoma-Marin Area Rail Transit project.

Using weights to counterbalance the 2.2-million pound structure, workers with Schimmick Construction delicately dropped the bascule bridge until it made contact with the river’s south bank.

“This is definitely a milestone,” said Matt Stevens, a spokesman for the SMART agency. “It’s one of the most significant parts of the system and one of the largest pieces of infrastructure we’ve had to build. It’s certainly one of the most visible parts.”

Until last Thursday’s pull down, workers were essentially constructing the bridge vertically as a 157-foot tower visible to commuters on the nearby Highway 101 bridge. The test closing of the bridge was the first time it spanned the river.

“It’s a demonstration of the successful completion of the bridge,” Stevens said. “It definitely bridges something now.”

Workers this week were scheduled to perform more tests and open and close the bridge using its motor. The tracks will be closed to rail traffic for the next two weeks while workers finish the project, including laying tracks across the new bridge, removing the old swing-span bridge and linking the new tracks with the old tracks, Stevens said.

SMART purchased the 30-year-old bridge from a railroad in Galveston, Texas for $4 million and shipped it to Mare Island, where it was partially reassembled and painted in SMART’s trademark McGlashan green. It will replace the 111-year-old Haystack Bridge, which will be sold for scrap after SMART received no offers to preserve the historic span.

In recent years, rail traffic on the North Coast has been light with only a few freight trains per week, and the old bridge sat mostly in the open position allowing unfettered river passage for boaters. But once SMART begins commuter rail service between Santa Rosa and San Rafael, which is expected to start late next year, trains will cross the bridge every 15 minutes during the morning and evening commute times, and the span will need to remain closed for most of the day, Stevens said.

The Petaluma River is the only spot on SMART’s 42-mile route where the tracks cross a navigable waterway, which is controlled by the Coast Guard. Under Coast Guard regulations, any boat can ask for the bridge to be raised. Stevens said SMART was working with commercial river operators to coordinate schedules so as not to disrupt the train service. Small craft and rowers can pass under the bridge, even in the down position.

The new span will be able to open in two minutes, instead of the seven that the current bridge takes. It will also allow for a 33-foot wider river channel, a benefit for commercial boat traffic, said John Riley, the bridge project leader for SMART.

“It’s going to be a much more reliable bridge,” he said.

(Contact Matt Brown at matt.brown@arguscourier.com. On Twitter @MattBrownAC.)