The Cambridge Railroad (also known as the Cambridge Horse Railroad) was the first street railway in the Boston, Massachusetts area, linking Harvard Square in Cambridge to Cambridge Street and Grove Street in Boston's West End, via Massachusetts Avenue, Main Street and the West Boston Bridge.
The company was chartered and incorporated May 25, 1853, and started construction September 1, 1855, opening the line to Central Square on March 23 or March 26, 1856 and later to Harvard Square. Streetcars were pulled by horses. From its incorporation, it was leased to the Union Railroad for 50 years, later passing under control of the West End Street Railway and the Boston Elevated Railway. The Red Line subway, opened March 23, 1912, now travels this route mostly underground, though the streetcar tracks remained for a while, possibly used by East Boston Tunnel (now Blue Line) trains until 1924 to travel to repair yards. The Red Line still travels over the Longfellow Bridge, successor to the West Boston Bridge, on the surface.
The line beat a rival company by buying secondhand cars from the Brooklyn City Railroad. For the first two months, no fares were charged, making the line very popular, with over 2,000 passengers per day within a week. When it started to charge fares, the public was outraged, many calling for the franchises to be revoked.