In order for the Green Line to get longer, it must first get a little shorter — for a little while.

The MBTA is closing the decrepit Lechmere station at the Green Line’s current northern terminus next spring while it builds a new station across Monsignor O’Brien Highway, meaning for the better part of a year the trolleys will end their run at North Station. The Science Park station will also be closed while the T conducts major repairs to the viaduct that carries the Green Line.

During construction — scheduled from May 2020 to April 2021— the Massachusetts Bay Transportation Authority will run shuttle buses between the old Lechmere location and North Station.

Moving the Lechmere station across the O’Brien is a crucial step in the $2.3 billion project to extend the Green Line to Somerville and Medford, to align the new stop with the right-of-way that will carry trolleys further north.

Long-delayed and often troubled, the dream to extend the Green Line is actually on its way to becoming reality. Further north, construction workers are relocating commuter rail tracks along the right-of-way to make way for new trolley lines. And travel around Somerville has been seriously disrupted by the closure of three bridges to accommodate the work. But despite some hiccups in the last year, officials say the project remains on schedule to be completed by the end of 2021.

Meanwhile, for the thousands of riders who use Lechmere, the price of progress will be a lengthy inconvenience. Some are pressing the state to make sure the shuttle buses have easy access to Boston along traffic-clogged Route 28 — ideally by dedicating a travel lane to buses. The highway carries more than 34,000 cars a day and is often heavily congested throughout the tangled nest of roads that feed into Boston’s West End.

“The O’Brien Highway gets very crowded at all times of day now,’’ East Cambridge resident Naomi Wheatley said at a recent meeting of the state’s transportation board. “I’d really like to see a dedicated bus lane. I fear that the walk from North Station to Lechmere might be shorter than a shuttle bus ride for the same distance.’’

The closure will also impact T riders from outside the nearby neighborhood. Lechmere is a significant bus hub, where more than 2,400 passengers depart buses each day, accounting for more than 40 percent of those routes’ overall ridership.

Some riders don’t expect the disruption to be too bad, as there are other transit options nearby. Orlan Rosas, who lives in Union Square, typically takes a bus to the Green Line. But he can also use the Orange Line. The small inconvenience will be worthwhile, Rosas said, because the expansion project will eventually bring a Green Line stop to his neighborhood.

The T’s shuttle bus plans are still being sorted out. It’s not yet clear whether shuttle buses will pick up riders at the current Lechmere bus terminal or somewhere else, or even where riders on regular bus routes in and out of the station will be dropped off. If existing bus routes still end at Lechmere, riders continuing on to Boston would be asked to transfer from bus to shuttle bus to train.

MBTA spokesman Joe Pesaturo said the agency expects to finalize the plans before a Jan. 13 board meeting.

The T has shut stations including Government Center and Wollaston in recent years for full renovations that have typically taken a couple years. Unlike those stations, Lechmere is located at the end of a line, and is changing location.

The new Lechmere will be elevated, with stairs and elevators leading to a waiting area. Concrete columns that will support the station are already in place at the site across the road.

The station construction adds to the huge amount of development in East Cambridge; the station is being built alongside the burgeoning Cambridge Crossing mega-development, already bringing thousands of new jobs and residents to the area. On the other side of Lechmere, there are big plans to redevelop the Cambridge courthouse tower and parts of the CambridgeSide Galleria.

But moving the station will force T riders coming from the East Cambridge side to cross the busy O’Brien Highway. That should become an easier crossing if, as expected, First Street is eventually extended to the NorthPoint development — cutting across the location of the current Lechmere station.

Once the closure goes into effect, the T will also be working around-the-clock for several months on a $62 million job to rehabilitate a major portion of the elevated viaduct that carries the Green Line over the Charles River and alongside Route 28. The agency was already planning to replace the portions closest to Lechmere as part of the Green Line project, but added the stretch between Science Park station and the Gilmore Bridge to take advantage of the Lechmere closure.

The work will improve the 110-year-old structure in the short term, shoring up arches and beams and allowing trains to move more quickly over it. But it also has longer-term ambition: making the viaduct strong enough to handle the next fleet of Green Line cars, which officials are designing to be twice as large as the current generation.

Adam Vaccaro can be reached at adam.vaccaro@globe.com. Follow him on Twitter at @adamtvaccaro.