The otherwise double-tracked Fitchburg Line has a short section of single track through downtown Waltham.

[3] The Fitchburg Railroad opened west to Waltham on December 20, 1843, then to Concord on June 17, 1844.

The Watertown Branch was extended to Waltham in 1851, meeting the Fitchburg mainline with a wye.

[9][10] Watertown Branch passenger service ended on July 9, 1938, though the line remained in use for freight until the early 20th century.

[11][1] The Massachusetts Bay Transportation Authority (MBTA) was formed in 1964 and began subsidizing B&M commuter service the following year.