A map inside an MBTA Green Line train showing the GLX extension through Somerville and Medford. Photo by Sam Drysdale | State House News Service

As the Medford branch of the $2.3 billion MBTA’s Green Line Extension makes its debut today, the long-anticipated transit project has already left its mark on the residential real estate market in Cambridge, Medford and Somerville.

An analysis of transaction data by The Warren Group indicates that residential condominiums located near the 4.7-mile route have fetched a premium in anticipation of the extension’s arrival.

The study shows that prices of condos located within a quarter-mile of GLX stations have risen faster than those elsewhere in the three cities. The new trolley line includes stations in East Somerville, Gilman Square, Magoun Square, Ball Square and College Avenue in Medford at Tufts University.

Between 2011 and 2021, median condo prices rose 129 percent within a quarter-mile of the future GLX stations, compared with 106 percent for those within a half-mile and 99 percent throughout the three cities.

The price appreciation near the GLX is difficult to quantify because median condo sales have risen sharply during the same period throughout Middlesex County, the research points out. But price growth has been highest in zip codes closest to downtown Boston, including the three communities directly affected by the extension.

Judy Sousa, a Realtor for Berkshire Hathaway HomeServices in Winchester, said the completion of the project adds one more selling point for nearby neighborhoods.

“People were excited that it might be coming, but people love anywhere they can walk to shops and restaurants and the post office,” said Sousa, a Medford resident. “And then you add the transportation component and it just makes the location even more desirable.”

A Metropolitan Area Planning Commission report in 2014 foresaw the effects of the project on real estate prices and the potential to accelerate displacement.

The report cited a similar uptick in values of properties in Somerville located within a half-mile of the MBTA’s Davis and Porter stations, resulting in a “proximity premium” of 27 percent compared with other properties in the city.

The potential for more rent increases and displacement prompted protesters from Community Action Agency of Somerville to picket this morning’s opening ceremony at the Medford/Tufts station.

The Warren Group analysis shows that GLX-proximate condos’ price premium has shrunk in recent years, indicating that the financial effects of the extension may have been “baked in” early in the project timeline.

National real estate data also shows faster price appreciation for lower-density suburban portions of major metropolitan areas, defined as population density below 5,000 per square mile, than more denser urban areas.

In 2015, a $1 billion increase in project costs prompted MBTA officials to delay the start of construction and subsequently redesign the project. The agency awarded a contract for the scaled-down project in 2017.

About 50,000 daily riders will use the new five-stop line to get between Boston, Somerville and Medford, according to the MBTA.

The Warren Group image