Haymarket North Extension

North of Wellington, the line ducks into a short tunnel under the former Medford Branch, then rises onto an embankment as it enters Malden.

[citation needed] Unlike the Washington Street Elevated (which was built at the same time and with a similar design), the Charlestown El was located very near Boston Harbor and the Mystic River tidal estuary, and was thus continually exposed to accelerated corrosion caused by salt air.

In 1965, a promised removal of the elevated structure was part of a compromise deal by Edward J. Logue to secure local support of a planned redevelopment project.

[3] After debate about how far the extension would run, it was decided that it would terminate at the north edge of development in Malden, rather than continue through a narrow pass between the Middlesex Fells and the Pine Banks and into the less-dense suburbs of Melrose and Wakefield.

In 1970, the MBTA made plans to develop a dual-mode diesel-electric railcar that could use the existing commuter rail tracks north of Oak Grove to allow through service between Boston and Reading.

[4] The plan was soon quashed by the state Department of Public Utilities, which objected to the safety risks of an electrical issue igniting diesel fuel in the downtown tunnels.

The tunnel sections on land were built using the cut-and-cover method; they consist of a 24-by-34-foot (7.3 m × 10.4 m) reinforced concrete box some 3–3.5 feet (0.91–1.07 m) thick.

[6] Construction of the Boston end of the tunnel unexpectedly required underpinning of North Station, an adjacent industrial building, and the Central Artery/Leverett Circle highway bridge.

Opening the first segment to Sullivan Square took 8+1⁄2 years – twice as long as expected – and the $180 million cost was over double original estimates.

The B&M refused to allow full construction access until the MBTA spend $18 million to purchase the Western Route from Somerville to Wilmington – a transaction not completed until September 1973 – and numerous B&M mainline and yard tracks had to be relocated.

Under that plan, without commuter tracks between Boston and Reading, all Haverhill trains would have continued to use the Lowell Line and Wildcat Branch.

A key piece of the development was an infill station at the site between Sullivan Square and Wellington to reduce the number of car trips generated.