Building a rail link between North and South station could cost at least $12.3 billion, double the initial highest amount expected, according to a draft study by the MBTA.
A study by the agency published last summer estimated that the rail link would cost between $4 billion and $6 billion.
The latest study by the agency found that the cost could range between $12.3 billion to $21.5 billion. The cost of the project, which would include burying commuter rail trains and Amtrak lines beneath Boston and offering enhanced service options, is reportedly comparable to the Big Dig tunnels.
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In 2012, state officials said the Big Dig cost $24.3 billion, the State House News Service reports.
Should the project go through, the new stations would be 115-195 feet deep, which would be deeper than the most subterranean MBTA station, according to the study.
Commuter rail ridership is expected to grow to 150,000 by 2040. Ridership would climb to 225,000 with a rail link and enhanced service, according to the study.
The study draft will be the subject of a public meeting Thursday.