The Green Line extension to Medford is opening Monday. Here's how it got there.

Monday is a day three decades in the making: The Green Line Extension into Medford will officially open, carrying its first riders at 4:45 a.m. GBH transportation reporter Bob Seay joined Morning Edition hosts Paris Alston and Jeremy Siegel to talk about the history and ramifications of this moment. This transcript has been lightly edited.

Jeremy Siegel: Let's talk a little bit before we get into the history about what we're selling here. What are the details of what is actually opening on Monday morning? Where are these stations? How many stations? What are we going to see?

Bob Seay: Well, this is the Medford branch of the Green Line extension. We already had the Union Square branch open up last March. That was a three-quarters-of-a-mile stretch of the new Green Line from Lechmere to Union Square in Somerville.

This one stretches about three miles from Lechmere up to Medford and Tufts University, and there are intermediate stations at Somerville, Gilman Square, Magoun Square and Ball Square. So it's going to provide five new platforms for passengers to be able to get back and forth on this Medford branch.

Paris Alston: And so this, of course, is coming off the heels of the Orange Line transformation. How big of a deal is this for the T, Bob? I mean, is there something for them to prove here, or is it just kind of a relief that it's finally happening?

Seay: Well, as they said, they've been able to prove that they can get big things done and they can get them done on time and within budget. And this is true of this extension, which is probably the biggest expansion of the T since 1987. And it's kind of interesting that Gov. Charlie Baker, as one of his last official acts, will be overseeing the opening of this new service.

And he, of course, earlier this week was down in Freetown, marking the South Coast Rail station completion in Freetown. That was the completion of the Fall River branch of that. So despite all of the other problems with the T, this is somewhat of a victory lap for the governor. And the MBTA's general manager, Steven Poftak, will be leaving his job when the governor steps down in early January. So this is a major milestone. This is something that people in Somerville, especially, have been waiting for since 1990. I don't think you were born back then.

Siegel: Neither Paris nor I were born back then. I mean, this is decades in the making. The origins go back a while. Give us some of the history behind this extension and the process to get it set up.

Seay: Well, it's really interesting. The first suggestion of extending the Green Line from Lechmere was actually 1922, 100 years ago. And nothing really happened.

Alston: OK, you were not born then.

Seay: [Laughs] So this actually began with the Big Dig, which is, of course, when they put the central artery, which used to be on an elevated highway through downtown Boston, underground in what is now the Tip O'Neill Tunnel, one of the largest tunnel projects in U.S. history. One of the problems with this was the increase in traffic and air pollution was of great concern. And the Conservation Law Foundation actually sued the state and said, you know, you're going to be violating the Clean Air Act by doing this and you really have to do something to mitigate it.

And there were many projects proposed. A lot of them have been completed, but one of the big ones was the Green Line extension. And one of the problems was it was so expensive, and it was going to be such a big project that it got delayed year after year after year. I think Congressman Michael Capuano once said he was never going to attend another groundbreaking, there were so many groundbreakings.

And the problem was the cost ballooned from $2 billion — with a B — to $3 billion. So they had to go back to the drawing board. And at times people thought it was dead. They thought it was never going to happen. But the good people of Somerville, led by their mayor, Joe Curtatone, would not give up. They kept the pressure on. They kept pushing, and they kept pushing. And finally, construction began back in 2018. And for many people, it was something that they thought would frankly never happen.

Alston: So now that it is happening, Bob, what's it going to mean for T riders and for potential new riders who might be serviced by this?

Seay: Well, the big thing is, a lot of people in the Somerville-Medford area who really had no other, rapid transit alternatives — in fact, Davis Square on the Red Line was the only one in Somerville for many years — this will enable them to get back and forth to Boston without driving and without having to take a bus. And this is going to be huge. I'd say 45,000 to 50,000 riders a day are expected to take advantage of this new Green Line extension. So it's going to mean a tremendous transit advantage and opportunity for so many residents.

Siegel: So this is a big change to transit. It's interesting hearing you talk about the Big Dig, which was obviously a huge change to transit, and that negative sides of that and how this Green Line extension came out of those negative downsides of it. Are there any downsides to an expansion of service like this?

Seay: Well, I'd say the one downside can be, and this has been noted for many years, the gentrification or displacement of residents along the route. Because what generally happens is property becomes a lot more expensive when, you know, people are selling condos or apartments that are near a T station. And so that has been a concern. Lack of rent control has made dealing with that difficult.

The MBTA did agree to maintain a bus route to try to help alleviate some of the pressure on people getting back and forth at the time and try to, you know, mitigate that. But it's a very difficult thing to do. And unless they do have specific multifamily housing units or affordable housing units that can be built nearby, which I believe there is one development now, it's something that inevitably sort of happens. So that's the downside.

The good side, of course, is you have this wonderful new transportation alternative. The downside is it will tend to increase rents and the cost of housing along the route. So those are things that have to be dealt with, still, in the future.