Haymarket North Extension  The Haymarket North Extension is a section of the Massachusetts Bay Transportation Authority's rapid transit Orange Line which currently constitutes the northern section of the line. It runs from North Station through an underground crossing of the Charles River (with the 2003-completed Leonard Zakim Bridge later built directly over it), then along the Haverhill Line right-of-way to Oak Grove station in Malden, Massachusetts. Built to replace the Charlestown Elevated and originally intended to be extended as far as Reading, it opened in stages between 1975 and 1977. Unlike the Washington Street Elevated (which was built as 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. The elevated was also unpopular with many local residents, as it was noisy and blocked out sunlight to Main Street. 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. With an elevated line thus politically infeasible, and a full-length tunnel too expensive, the project was designed to use existing railroad rights-of-way with limited tunneling. A groundbreaking was held on September 22, 1966. The extension replaced the Orange Line portion of the Canal Street incline, where the old El came to the surface north of Haymarket, with a tunnel under Canal Street to a new underground station at North Station, replacing the elevated station. The tunnel continues north under the Charles River to Charlestown, with an aboveground station at Community College that roughly replaced the former City Square and Thompson Square stations. The extension runs on the surface under the Interstate 93 elevated viaduct - constructed at the same time - and crosses the Haverhill Line and Newburyport/Rockport Line on a flyover, touching down at Sullivan Square (which replaced a nearby El station of the same name). The Charlestown El closed on April 4, 1975, and the new segment from Haymarket to Sullivan opened on April 7. The next section, from Sullivan over the new Edward Dana Bridge to Wellington, opened on September 6, 1975. The Wellington Shops for railcar maintenance, which replaced the Sullivan Square Shops and later the Forest Hills Shops, were opened with this extension. From Sullivan northwards, the extension runs entirely along the Haverhill Line right of way, which substantially lowered land acquisition difficulties before construction.