Weekend travel from Cambridge to Boston is about to get harder for a few months, as a complicated construction project will shut down weekend T service over the Longfellow Bridge.

For now, the Red Line is still rumbling over the Longfellow Bridge. But it’s coming to a stop here this weekend, and for nine more upcoming weekends.

“What you're looking at is some of the old infrastructure," said John McInerney of the state Department of Transportation as he lead a tour of the massive construction project on Wednesday. "The arches there, the curved piece is what’s staying, the structural steel above it's all being replaced.”

It’s an early 1900s bridge, McInerney said, and it was time for an overhaul. “It was just due," he said. "The inspection ratings and the engineers that review these bridges on a periodic basis — it was time. It's just a massive job. And it was time to get it reconstructed.”

John McInerney of MassDOT on the Longfellow Bridge Wednesday Craig LeMoult/WGBH

If it feels like the Longfellow has been under construction for a long time, that’s because it has.

Work started in the spring of 2013. It’s not a simple job.

“On a scale of complexity, I would say it's up there in the 8, 9 or 10 range,” McInerney said. That’s because they needed to keep the bridge open for all kinds of uses. “It's got vehicular traffic, train traffic, bike traffic, pedestrian traffic. And the phasing of the job has been very challenging over the course of the duration.”

And the historical nature of the bridge has made things more challenging. They restored its distinctive towers, with their salt and pepper shaker shape. And Chris Keough with the contractor on the job, White Skanska Consigli, said the historical commission told them they couldn’t use the bolts they normally would on a job like this. They had to use the same kind of rivets the bridge was originally built with.

“In the old days it was essentially hot rivets that have a connection to make between two pieces of steel, and they'd have a heated rivet and they'd drive it through the opening, and then it would it would it would cool off," Keough said. "And there you go. There’s your connection.” But nobody actually knew how to do that anymore, so they had to specially train workers for the job.

Construction work on the Longfellow Bridge Craig LeMoult/WGBH

This weekend, they’ll do the work to move Red Line trains from temporary tracks they’ve been on to more permanent tracks.

Todd Johnson of the MBTA says when they’re done, commutes should speed up a bit.

“Before the construction project, the speed over the bridge and continuing through the project has been 25 miles per hour," he said. "When the construction project is completed we will be resuming the speed on the Red Line trains over the bridge back to its 40 mph speed, which we haven't seen in a few years.”

In the meantime, though, shuttle buses will be running from Kendall to Park Street on the weekends. After this one, there are nine more to go – not including the weekends of the Head of the Charles regatta and Thanksgiving. In the end, officials estimate the price tag for the federally funded project should come in under the original estimate of $303 million. And the Longfellow should be fully open for all traffic by next May.