page_title: Haymarket North Extension
section_title: Construction
hierarchical_section_title: Haymarket North Extension / History / Construction
caption_attribution_description: English: Grade beams are constructed for the platforms of Wellington station around 1974
context_page_description: 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, 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.
context_section_description: The tunnel under the Charles River was constructed as an immersed tube, with steel tunnel segments sunk into a trench and filled with concrete using a tremie. 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. Transition sections, 70 feet (21 m) long with 4-foot (1.2 m)-thick walls, connect the land and underwater segments.
  The Charlestown Elevated closed on April 4, 1975, and the new segment from Haymarket to Sullivan opened on April 7. This segment was a direct replacement for the El: North Station and Sullivan replaced nearby elevated stations, while Community College roughly replaced the former City Square and Thompson Square stations. 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. An additional segment to the elevated Malden Center, which includes a platform for the Haverhill Line, opened on December 27, 1975.  The sixth and terminal station, Oak Grove, opened on March 20, 1977.
caption: Wellington station under construction
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