The Haymarket North Extension is an underground crossing of the Charles River which carries the Massachusetts Bay Transportation Authority's Orange Line. The term is also applied to the rerouting of the northern section of the Orange Line, which opened in 4 stages starting on April 7, 1975 and concluding on March 20, 1977.[1] The rerouting moved the north end of the Orange line into a new tunnel under the Charles, then along the Haverhill Line right-of-way to Oak Grove. It replaced the Charlestown Elevated, which used the Canal Street Incline and was demolished later that year.[2] An underground station at North Station and a surface station at Sullivan Square replaced elevated stations located nearby; the other 4 stations were new locations.
The extension was originally to continue further north to Reading and replace commuter rail service entirely between Boston and Reading. However, that extension was voted down by residents of reading. Under that plan, without commuter tracks between Boston and Reading, Haverhill trains would have solely used the Lowell Line and Wildcat Branch - a routing used today only for a handful of rush hour trains.
An infill station at Assembly Square is under construction and planned to open in 2014.
From north to south, the stations added or rebuilt for the extension:
| Station | Opening date[1] | Notes | |
|---|---|---|---|
| Oak Grove | March 20, 1977 | ||
| Malden Center | December 27, 1975 | Transfer to Haverhill Line | |
| Wellington | September 6, 1975 | ||
| Sullivan Square | April 7, 1975 | Separate from elevated station it replaced | |
| Community College | April 7, 1975 | ||
| North Station | April 7, 1975 | Replaced elevated station; rebuilt in 2005 for Green Line access |
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