What became the E branch was formed from portions of several streetcar lines. The first of these was the West Roxbury Railroad, a horsecar line opened on November 14, 1857 in what was then West Roxbury. It ran from to Jamaica Plain along Lowell Street (now Columbus Avenue), Centre Street, and South Street. The line was quickly leased by the Metropolitan Railroad, which operated through service between Jamaica Plain and downtown Boston using its line on Tremont Street. Travel time for the -long line was over an hour with a ten-cent fare. A two-track carhouse with a waiting room was located at the terminus of the line at Jamaica Street in Jamaica Plain. (The Metropolitan's main carhouse and barn was located at Roxbury Crossing.) In 1858, the Metropolitan opened a connector between Lowell Street and John Eliot Square, allowing Jamaica Plain cars to also use the Washington Street line (via ) to reach downtown. The railroad also rebuilt the Jamaica Plain line that year to accommodate heavier ridership, including the double-tracking of some portions.