The Lechmere Viaduct is a concrete arch bridge connecting the West End neighborhood of Boston to East Cambridge, Massachusetts.

The first streetcars crossed the bridge in revenue service on June 1, 1912, shortening the ten-minute trip from Lechmere Square to the subway to just three minutes.

This prevented traffic delays affecting the surface cars from propagating into the subway, and allowed the underground service to use three-car trains, which were too long for safe street-running.

[2] In 1935, a citizen's group proposed to widen the viaduct into a four-lane expressway to accommodate an extension of the Mohawk Trail; transit service would have been rerouted into a new parallel subway.

A new portal was built off Martha Road, with a new concrete bridge connecting to the viaduct just south of Science Park station.

[10] Only the steel section of the viaduct north of the Gilmore Bridge was replaced; the main concrete span was restored to support Green Line Extension service and future Type 10 vehicles.

[15] However, the work did not have the intended effect, and a multi-year long slow-zone was put in place that continued to limit speeds to 10 miles per hour (16 km/h).