In the middle of the night, as most of the D.C. suburbs are sleeping, construction workers are excavating a tunnel beneath eastern Silver Spring.

During the day in downtown Bethesda, other workers are digging — and soon will begin blasting — a massive elevator shaft alongside busy Wisconsin Avenue.

In College Park, students walking to class at the University of Maryland navigate rows of orange construction barrels in the heart of campus.

More than a year since the ceremonial shovels broke ground, construction on Maryland’s light-rail Purple Line has ramped up along its 16-mile path. While train tracks won’t appear until early 2020, the $2.4 billion project is in full swing throughout parts of Montgomery and Prince George’s counties.

But what makes a light-rail line ultimately successful — its ability to connect densely populated areas — also makes it challenging to build along congested roads and through crowded communities.

Carla Julian, spokeswoman for the team of companies designing and building the line, said project officials are “very sympathetic” to residents beginning to complain about vibrations and loud noise. She said workers have remained within noise and vibration limits set in the contract with the Maryland Transit Administration.

Even so, Julian said, “We’re constantly monitoring that. Unfortunately, it’s such an urban area that we’re right on top of people.”

Julian said the state’s “goal” remains for the line to begin carrying passengers in late 2022, despite a year of legal delays.

The transit project has attracted national attention as one of the first in the country built via an extensive public-private partnership. A team of private companies is helping to finance construction and will design and build the line before operating and maintaining it as part of a $5.6 billion contract over 36 years.

The Purple Line also is one of the first U.S. rail projects to connect suburbs without requiring riders to travel through a city center. Maryland officials say it will provide faster and more reliable east-west transit than buses and spark redevelopment around its 21 stations.

Most of the work underway is focused on cutting trees and moving underground utility lines, including along roads, project officials said. That means more lane closures — translation: traffic backups — as work progresses.

“This is a huge utility job,” said Scott Risley, the design-build team’s project director.

Most of the early work is in Montgomery. In downtown Bethesda, workers are digging a 150-foot shaft for six elevators that will carry passengers between the Purple Line and Metro’s underground Red Line station there.

When blasting begins in October, project officials said, specially fortified trucks will bring in the explosives daily from West Virginia because there will be too many to be stored safely on site.

Jean-Marc Wehrli, manager of tunneling work, said motorists on Wisconsin, adjacent to the underground blast site, “probably aren’t going to feel anything.”

Metro riders in the underground Bethesda station might hear noise and feel vibrations from the explosives, project officials said, but workers will try to limit blasting to times when Metro is closed.

Heading east, most of the trees along the four-mile Georgetown Branch Trail between downtown Bethesda and Silver Spring have been cut down, Julian said.

The trail’s closure during construction has required cyclists to use a detour along roads. However, some say, the construction boom in downtown Bethesda has left many sidewalks closed.

Anna Irwin, founder of the Bethesda Bike Now Coalition, said she sometimes finds sidewalks on both sides of a street blocked with orange barrels, forcing her and her 6-year-old daughter to venture into traffic on their morning rides to school.

“It’s gone from dangerous and bad to absolutely unacceptable and ridiculously unsafe,” Irwin said. “Someone needs to step up and realize that pedestrians and bikers need to be protected while all this work is going on.”

Tim Cupples, Montgomery’s coordinator on the Purple Line project, said the county is working with developers and the state to ensure that sidewalk closures are better coordinated.

Some of the biggest impacts are being felt near the 1,000-foot tunnel being dug round-the-clock in the Long Branch area of Silver Spring, where the terrain is too steep for trains to use roads.

Some nearby residents say they hear almost constant clanging, grinding noises and beeping backup alarms from 7 a.m. to 7 p.m.

Donna McLaren’s living room, which she said used to look onto “beautiful blossoming trees,” now overlooks a massive construction site at what will be the tunnel’s western portal.

McLaren, who has cancer, said she uses ear plugs and puts a pillow over her head to try to sleep after chemotherapy sessions, but it’s too loud. She said she can’t afford to move from her low-rent apartment building.

“I woke up today with my heart racing and the walls shaking, like it was an earthquake,” she said.

Julian said the project offered to put up McLaren in a nearby hotel during the loudest part of the construction, but she declined.

In the Lyttonsville neighborhood of Silver Spring, a bridge used by an estimated 10,000 vehicles daily on Lyttonsville Place has been torn down and is being rebuilt. It’s scheduled to be finished in late December, officials said.

Cupples said the county, anticipating cut-through traffic on nearby streets as motorists looked for ways around the closed bridge, has stepped up police enforcement, repainted crosswalks and lowered speed limits to 15 mph from 25 mph.

Charlotte Coffield, president of the Lyttonsville Community Civic Association, said she’s concerned that traffic through the neighborhood — much of it speeding — has more than doubled since the bridge was torn down.

“I think they’re trying to address our concerns,” Coffield said of the county. “This isn’t going to last forever.”

East of downtown Silver Spring, there is concern among parents of children who cross Wayne Avenue — where a Purple Line station will be built — to reach Silver Spring International Middle School and Sligo Creek Elementary School.

So far, residents said, parents haven’t voiced any major concerns beyond bemoaning the loss of 20 or so towering trees that were cut down over the summer.

“It’s permanently changed the landscape,” said Chris Richardson, president of the Park Hills Civic Association. “The heat was pretty punishing without those trees. We’re taking a lot on the chin for the Purple Line.”

At the University of Maryland, project officials closed the eastbound lane of Campus Drive last month. Students now walk around bright orange and white construction barrels lining the road west of the “M” circle, where underground utilities will be dug up and moved.

Carlo Colella, a university vice president, said construction is expected to last three years, but he’s looking forward to the five Purple Line stations that will be on or near the campus.

“I’m very enthusiastic about this,” Colella said. “We’re very, very excited to see it finally come to fruition. This will be a transformative project.”

People who want a heads up about Purple Line construction and traffic impacts can sign up for text or email alerts at ­