The price tag for constructing a long-discussed north-south rail link between Boston’s North and South stations is now estimated at $4 billion to $6 billion, much less than prior estimates. That’s according to a group of Harvard Kennedy School (HKS) students and faculty who calculated current project costs from federal data on modern tunneling technologies.

The study could bring fresh momentum to efforts to persuade state lawmakers to move forward with the project, which analysts say would greatly improve downtown traffic flow for those commuting through the city.

A North-South Rail Connector for Boston

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A group of Harvard Kennedy School students and faculty calculated that a Downtown Boston rail link could be built more cheaply than previously thought. Video by Jacob Beizer

According to the new research, the project could cost $6 billion for a “maximum build,” which would include four tracks, two tunnels, and three stations. The price tag drops to an estimated $3.8 billion for a “minimum build,” which consists of two tracks, one tunnel, and two stations. The figures are inflated to 2025 dollars, which is the assumed midpoint of construction.

The estimates were calculated using a robust set of historical project data from the Federal Transit Administration. The figures were substantially lower than those calculated during the last major study of the link project, conducted by the Massachusetts Bay Transportation Authority in 2003.

“The difference in the estimates may be due to advances in technology that have reduced the cost of tunnels in similar infrastructure projects,” the study authors write. “These estimates suggest that the economic case for the project might be significantly more attractive than earlier assumed.”

The authors are Laura White, M.P.P. ’18; Jean-Louis Rochet, M.P.P. ’17; Pete Mathias, M.P.A. ’17; Kate O’Gorman, M.P.A. ’17; and Linda Bilmes, Daniel Patrick Moynihan Senior Lecturer in Public Policy at HKS. The authors suggest that their findings could spur additional research to measure and confirm the potential costs and benefits associated with the link.

“We also recommend further research into effective contract models, management structures, and comparable tunneling projects that have experienced favorable cost outcomes,” the authors say. “The experience of the Western European tunneling projects examined in the regression analysis suggest that there may be areas for cost savings that are not anticipated by this study. Incorporating best practices from these projects may further improve the cost and risk profile of the link proposal.”

A north-south rail link was initially proposed more than a century ago, and it has received added political support in recent years, including from U.S. Rep. Seth Moulton ’01, M.B.A.-M.P.P. ’11, who represents Massachusetts’ 6th Congressional District. The state Department of Transportation’s Office of Transportation Planning recently awarded a $1.5 million contract for a new study of the link.

The research project was conducted under the supervision of Bilmes and under the auspices of the Greater Boston Applied Field Lab. The lab is part of the Kennedy School’s Rappaport Institute for Greater Boston, with funding from the Phyllis & Jerome Lyle Rappaport Foundation.