Starting this weekend, Sound Transit light-rail passengers will shimmy through crowds and detours for 10 weeks while workers build new track connections from Seattle to the Eastside.

Pioneer Square Station will be the last stop for trains arriving from both the north and south, so an estimated 30,000 daily riders must change trains by walking across a concrete median there to continue traveling toward the University of Washington or SeaTac.

Train operators will try to synchronize, so trains from the north and south arrive together to exchange passengers. Riders are encouraged to allow for up to 30 minutes delay, in the first few days.

These temporary, wishbone-shaped train paths will give contractors room to attach new rails at the International District/Chinatown Station. The new tracks will lead to the former express lanes of Interstate 90, where the world’s first transit rail on a floating bridge is being installed.

As part of the project, the downtown transit tunnel will fully close this weekend and the weekends of Feb. 8-9 and March 14-15. Buses will shuttle people between Stadium Station, the four downtown stations and Capitol Hill Station.

Sound Transit calls the project Connect 2020.

The track-connecting work could have been deferred until 2022, because the Eastside extension reaching Mercer Island, Bellevue and Overlake won’t open until 2023. But the agency chose to do it now to inconvenience fewer riders.

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Trains now handle about 80,000 daily boardings, but that number is expected to jump to 130,000 when U District, Roosevelt and Northgate stations open in 2021.

Trains will be lengthened to four railcars, for a standard capacity of nearly 600 passengers, instead of the usual mix of two- and three-car trains. (As many as 800 people could fit in a four-car train when packed, such as after a stadium event.)

But the trains will run less frequently, every 12 minutes all day, because shorter intervals would mess up the timing at Pioneer Square. Normal frequency varies from six minutes at peak to 10 minutes midday and 15 minutes late at night. The 12-minute intervals provide less capacity overall and more crowded trains at times.

During construction, trains will share a single track through Westlake, University Street and International District/Chinatown stations. Only one boarding platform at each stop will serve both train directions.

More railcars aren’t available to boost capacity. The transit board, surprised by fast ridership gains mid-decade, didn’t order new, spacious Siemens railcars, intended for the 2021 extension to Northgate, early enough to bring them into service now.

Four temporary speakers were installed in Pioneer Square Station south mezzanine, so staff can issue instructions to passengers below or announce when the next train is due, as conditions change. The tunnel’s permanent public-address system is limited to broadcasting prerecorded messages.

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Railcars could be 15% more crowded than usual, Sound Transit says. Many will exceed the standard load of 148 people per railcar and occasionally hit 200, said agency spokesman David Jackson. At that point, he said, some people will choose a different mode of travel, or reschedule their trips.

Some travelers will get off at a different station than they normally would and walk farther than usual. Taxi and ride-hailing trips are expected to increase, adding to traffic congestion. The bike-rental company Jump will offer a 20% discount for users who apply the code JUMPCON20.

Metro will add 20 buses on five north-south routes, giving people more frequency and options. These are the 7 to Rainier Avenue South, the 36 to Beacon Hill, the 70 through Eastlake to the University District, the 48 that serves both University of Washington and Mount Baker stations, and the 49 to north Capitol Hill.

Riders won’t be allowed to bring bicycles into or through Pioneer Square Station, rider-alert posters say. The ban aims to enhance safety and reduce crowding as riders move between trains. Recently, 94 bike lockers were added at UW, Sodo and Rainier Beach stations.

However, the advocacy nonprofit Rooted in Rights is trying to assure that people with mobility limitations can still bring bikes and scooters through Pioneer Square Station, program director Anna Zivarts said.

“In theory Sound Transit agrees with this, but it will be interesting to see how this works in practice,” she said. Sound Transit spokesman Jackson said the agency “we’ll have to take people’s word” that they need a bike or scooter to move about.

Zivarts predicts that increased crowding will pose problems for some people, and that transit staff will have difficulty communicating about the station changes to people for whom English is a second language.

Another source of tension will be unreliable 30-year-old downtown escalators and elevators. In mid-December one-fourth of them weren’t operating.

“An escalator at Westlake badly malfunctioned while passengers were on it, and we were fortunate there were no injuries,” Sound Transit CEO Peter Rogoff told the agency board last month.

Sound Transit will take ownership from Metro of stations in the transit tunnel later this year, and plans to launch a $52 million escalator and elevator rebuilding program.

As of Tuesday morning, all Pioneer Square Station escalators and elevators worked, as pledged last week by Bill Bryant, Metro deputy director for service development.

Maintenance firm KONE has reduced its work backlog the last two weeks. Metro says there were no incentive payments or penalties, except goals the agency set. “Nothing has changed. We remain vigilant in our goals to help keep our customers’ equipment in safe, optimal operation,” said Patrick O’Connell, KONE marketing director in Chicago.

Still, one escalator at International District/Chinatown Station, one escalator and one elevator at University Street Station, and three of the 16 escalators and the central elevator at Westlake Station were not working on Tuesday.

Contractors already have laid the new tracks entering International District/Chinatown Station, ready for final connections to the mainline. In the station median, workers are adding a central “pocket track” where empty trains could wait to scoop up stadium crowds, or switch directions to dodge a downtown or Sodo blockage.

That should make the 21-mile light-rail system more resilient.