SEATTLE — They weigh between 150,000 to 175,000 pounds and run 140 to 150 feet long.

If you’ve driven between Lynnwood and Seattle via I-5 in the past year, you’ve probably seen them, but they only move at night.

They’re the big, gray girders that connect the newly constructed columns lining the interstate. Placing them requires two cranes and a crew of 10 to 12. As crane operators hoist the beams into the sky, workers line up exactly where they need to land on the capped columns.

“They actually set up a laser to say, ‘Here’s where the girder goes,’” said Ginger Ferguson, a Sound Transit construction manager. “I can’t believe they get them on there.”

The beams, made of concrete and rebar, come from Tacoma and are so large that some can’t travel through the tunnel under the Seattle Convention Center, so they take I-405. And they must arrive facing the correct direction, because they’re too heavy to flip.

All of this happens under the cover of darkness to give the transport trucks and construction crews the space they need, Ferguson said.

By 2024, tens of thousands of people will ride above the girders each day when the Lynnwood Link of Sound Transit’s light rail system is finished. A trip from Lynnwood to Seattle will take 27 minutes, regardless of I-5’s traffic.

“The shape of the alignment becomes clearer with each passing day,” CEO Peter Rogoff said in a news release. “The completed extension will be a game-changer for anyone traveling between King and Snohomish counties.”

In the past year, more than 85 columns and 25 girders have gone up past Northgate toward Snohomish County as construction crews work to extend track north.

Workers have to place about 100 more columns, and each column gets three to four girders.

“There’s still a lot more work to go — it just won’t look quite as dramatic after we get the girders done,” Ferguson said.

Last week, the project reached a milestone when crews placed the girders that connect the Northgate station — set to open next year — with the southernmost column for the Lynnwood link.

With girders up in some places and yet to be installed in others, their placement can seem random. That’s because they have different shapes and alignments, Ferguson said.

Before the first columns and girders could be set, crews had to pull trees, clear soil, remove old utility pipes and demolish some roadways and buildings.

During the winter and spring months, fleets of tankers were on site to collect water and contain the runoff, to avoid erosion.

When completed, the Lynnwood Link and other extensions, including new stations in north Seattle, Shoreline and Mountlake Terrace, will more than double the length of the region’s light rail system.

Joey Thompson: 425-339-3449; jthompson@heraldnet.com. Twitter: @byjoeythompson.