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.

History

Need
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.

Route and construction

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. An additional segment to the elevated Malden Center, which includes a platform for the Haverhill Line, opened on December 27, 1975. 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. The sixth and terminal station of the Extension, Oak Grove, opened on March 20, 1977.

Reading plans and third track
The extension was originally planned to continue further north to Reading and replace commuter rail service entirely between Boston and Reading.  Under that plan, without commuter tracks between Boston and Reading, all Haverhill trains would have continued to use the Lowell Line and Wildcat Branch.  That routing was used for all Haverhill service between 1959 and its termination in 1976.  (When Haverhill service returned in 1979, it was routed via Reading; only a handful of rush hour trains, plus Amtrak's Downeaster service, still use the Wildcat Branch.) 

A third express track was built as part of the extension; it would have been extended north to provide express service to Reading. The  actually built run from just south of Community College to just north of Wellington. In September 1972, the MBTA announced the previously private decision that due to a lack of immediate funding to continue the line past Oak Grove, express service would not be run, as the time savings from Malden would be limited and the third track space through Malden was needed for commuter rail. In 1982, a General Accounting Office report criticized the MBTA for spending $2.3 million in federal funds on the third track, though the MBTA argued that the inability to run express service was due to political forces beyond MBTA control.

The third track is occasionally used for other purposes like testing new equipment and rerouting trains during extreme weather conditions. In 2007, it was used to test new Blue Line cars. During the Assembly station project, outbound trains were shifted to the third track and inbound trains to the normal outbound track to allow construction work on the station.

Architecture
Like the stations of the South Shore Line (the Braintree branch of the Red Line), built in the same period, the original Haymarket North Extension stations were constructed in a Brutalist style, with angular concrete forms.  Community College and Sullivan Square, enclosed under the I-93 viaduct, are primarily rectilinear forms; the later stations incorporate angled forms for staircases and elevators as well.

Assembly station
In the early 2000s, Somerville began planning the Assembly Square development, built on the abandoned site of the former Somerville Assembly automobile plant.  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.  After two years of construction, Assembly station opened on September 2, 2014.  It was the first fully new station on the MBTA subway system since 1987, and the first infill station since Science Park in 1955.  Assembly station has an island platform between the two main tracks; during construction, outbound trains were diverted onto the normally-unused third track, and inbound trains onto the outbound track.  The station has a boxy form, with a similar shape to the original Extension stations but built in a different style.

Station listing
Stations are listed in order from the northern terminus successively to the south.