The Cambridge Railroad was granted permission in 1860 to run horsecar tracks over the Craigie Bridge from East Cambridge to Boston. Service began from Harvard Square along Cambridge Street to Lechmere Square around 1861, and from Lechmere Square over the bridge into Boston by 1862. In 1864, the company built a line from Union Square in Somerville along Somerville Avenue and Bridge Street to the existing tracks at Lechmere Square, allowing through service from Elm Street (now Davis Square) to Boston. The Cambridge Street line was converted to electric streetcar service by Cambridge Railroad successor West End Street Railway in 1891; the Somerville Avenue line and an 1888-built line to Davis Square via Highland Avenue were electrified and extended to Clarendon Hill in 1893–1895.
