The Haymarket North Extension is an underground crossing of the Charles River

Charles River

The Charles River is an long river that flows in an overall northeasterly direction in eastern Massachusetts, USA. From its source in Hopkinton, the river travels through 22 cities and towns until reaching the Atlantic Ocean at Boston...

 which carries the Massachusetts Bay Transportation Authority

Massachusetts Bay Transportation Authority

The Massachusetts Bay Transportation Authority, often referred to as the MBTA or simply The T, is the public operator of most bus, subway, commuter rail and ferry systems in the greater Boston, Massachusetts, area. Officially a "body politic and corporate, and a political subdivision" of the...

's Orange Line

Orange Line (MBTA)

The Orange Line is one of the four subway lines of the Massachusetts Bay Transportation Authority. It extends from Forest Hills in Jamaica Plain, Boston in the south to Oak Grove in Malden, Massachusetts in the north. It meets the Red Line at Downtown Crossing, the Blue Line at State, and the Green...

; the term is also applied to the rerouting of the Orange Line in 1975. 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

Oak Grove (MBTA station)

Oak Grove Station is an MBTA station on the Orange Line, located in the northern part of Malden, Massachusetts near the intersection of Winter Street and Main Street, and adjacent to the city of Melrose....

. It replaced the Charlestown Elevated

Charlestown Elevated

The Charlestown Elevated was a former link of Boston's Orange Line rapid transit line that ran from a portal at North Station, near the old Boston Garden, to the city of Everett, Massachusetts...

, which was demolished that year.