On June 25, 2004, service over the viaduct was suspended as the Causeway Street Elevated was removed, and the surface and elevated platforms at North Station were consolidated into a new underground "superstation". This left the Lechmere Viaduct and Lechmere Elevated as the sole remaining elevated lines in Boston. A new portal was built off Martha Road, with a new concrete bridge connecting to the viaduct just south of Science Park station. Service to Lechmere resumed on November 12, 2005. The incline from the 2005-built tunnel to the Lechmere Viaduct is the steepest grade on the entire MBTA subway system. The viaduct was again closed from April 30 to November 4, 2011, as Science Park station was rebuilt with accessible platforms and new elevators.
