The Causeway Street Elevated was a part of the MBTA's Green Line in Boston, Massachusetts that ran roughly northwards from Haymarket, emerging from the Canal Street Incline in parallel with the Charlestown Elevated tracks, then ran westwards after splitting from the Charlestown Elevated's parallel right-of-way for a short distance around the former Boston Garden — adjacent to and along the southeast-facing side of the older indoor sports venue's exterior (itself replaced in the mid-1990s by today's TD Garden) through the Green Line's North Station elevated stop, and resumed a north-westwards direction past North Station and the Garden, ending at Science Park. As the name suggests, the Elevated ran above ground, until it was closed in June 2004, being replaced by an underground tunnel as part of environmental remediation to compensate for increased air pollution caused by the Big Dig.
Today, the Green Line emerges from the underground tunnel about halfway between North Station and Science Park, where it connects via ramp directly to the Lechmere Viaduct, the Green Line's only remaining elevated track section. The MBTA has no plans to relocate any of the remaining track on the viaduct to an underground route.
The removal of the Elevated freed up land near the Big Dig and lowered noise levels associated with the trains' characteristic screech as they rounded sharp turns in the track.