MBTA Green Line Extension program manager John Dalton talks to reporters at the final stop of the Green Line Extension, Medford/Tufts Station, which will open to passengers on Monday. Photo by Sam Drysdale | State House News Service

All aboard to Medford.

Residents in cities north of Boston only have to wait a few more days, the last in a decades-long span of promises and anticipation, before they can hop on a Green Line trolley into the state’s capital city.

The second, longer stretch of the Green Line Extension opens for business at 4:45 a.m. on Dec. 12, and will carry an estimated 50,000 daily riders between Boston, Somerville and Medford, according to MBTA officials. Residents of greater Boston’s northern communities have relied for decades on buses or long trips to reach the Orange and Red Lines to get into downtown Boston.

Riding from the new northernmost stop on the Green Line, Medford/Tufts, to the current and longtime Green Line terminus, Lechmere Station, should take about 15 minutes – including time to stop at all five new stations along the extension, said MBTA director of communications Joe Pesaturo.

From the T’s only other stop in Medford, Wellington Station on the Orange Line, to the closest downtown Boston stop, North Station, the MBTA benchmarks an average trip time of eight minutes, according to transit nonprofit TransitMatters’s data dashboard.

Trains on the new extension will travel at an estimated 40 miles per hour between stations, MBTA Green Line Extension program manager John Dalton told reporters on a trolley heading north on the new tracks in a “test ride” preview Wednesday afternoon.

“We’ve had some pretty strict management of the budget, which is not always easy,” Dalton said. “It made some people frustrated when they wanted something to be added to the project and we held a pretty firm line to say we couldn’t do that, just so when we got to this point, or encountered surprises … or COVID-19, we could absorb those impacts without really tripling the budget.”

It is too early to say if any of the money dedicated to the GLX budget will flow back to the state, Dalton said. The MBTA needs to finish its “normal closeout process” before it gets a full financial picture, which Dalton said he hopes will be completed by summer 2023.

Construction began in July 2018, but COVID-19 caused delays.

“Our supply chain was directly impacted. A job like this with materials from local suppliers, some supplies from around the world, and you have a global pandemic – regardless of where the hotspot is in the world, we’re going to feel it,” he said.

Part of the closeout financial process for the MBTA in months ahead will be calculating how much those delays cost.

The design of both branches of the GLX – the E Branch opening to Medford next week and the D Branch to Union Square completed in the spring – allows for future extensions, according to Dalton.

“There’s nothing physically that this project put in place to prohibit a future extension, so you know, there would be a lot of work to be done at either branch, but this project didn’t make it any harder than it would be already,” he said.

As for the challenges workers have faced, the program manager said they were “very fortunate” with safety incidents.

“We had no fatalities, which is a good thing. A lot of projects can’t make that claim after a job like this,” he said.

While there were some injuries on the job, they were “well below” the threshold for an unacceptable level of incidents based on industry safety standards, Dalton said.

In addition to pandemic-related delays, there are some unforeseen circumstances that are impossible to plan for, he said, like when workers unearthed an old rail car near what is now East Somerville station.

About 16 years after the MBTA agreed to complete the GLX and around three decades since the idea was first introduced, local officials plan to celebrate the new stations’ opening next week.

Somerville Mayor Katjana Ballantyne and other city officials are planning a rush hour celebration at Ball Square Station on Dec. 12 with a band and free opening day memorabilia.

Lt. Gov. Karyn Polito said Tuesday that the Baker administration will also “be talking about GLX” on the day of its launch.

As for the first ride, MBTA General Manager Steve Poftak said in November, “I know there’s a universe of enthusiasts for this type of thing, so I look forward to seeing you all at approximately 4:45 a.m.”