The Red–Blue connector is a proposed extension of the Blue Line from west under Cambridge Street, providing a direct transfer between the Red and Blue lines. The project was first proposed in 1924, and was returned to consideration in the 1978 update to the Program for Mass Transportation. In 1991, the state agreed to built the project by 2011 as part of the settlement of a lawsuit over auto emissions from the Big Dig project. This commitment was changed to design only in 2007–08 and lifted entirely in 2015.