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The long-anticipated Green Line extension, which will continue the country’s oldest subway line from Lechmere Station in Cambridge into Somerville and Medford, will cost more than $2 billion, we learned this week.

While not the most expensive transit project in Massachusetts history — that designation goes to the Big Dig, which logged $15 billion in construction costs — the extension’s price tag could appear a bit steep for just 4.7 miles of new service. And the estimated cost has only grown as the project has been delayed, time and again. It’s now projected to be completed between 2017 and 2020.

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So what’s included in the price tag?

A lot of contingency costs, to cover any new delays, just in case. One of the biggest line items is the purchase of 24 new trolley cars, at about $165 million. And then there is all the actual labor, and construction of new stations and tracks.

But the new estimate also includes some changes to the project, including a community path in Somerville that will mostly run alongside the new route.

Related reading:

Federal officials pledge nearly $1b for Green Line extension

Green Line extension costs is going up several million dollars

Will the Green Line extension make Somerville real estate go crazy?


Nicole Dungca can be reached at nicole.dungca@globe.com. Follow her on Twitter @ndungca.