The Boston College branch is also known as the Commonwealth Avenue line. Like the C line, it features lots of stops spaced closely together, to the point where the biggest complaint among riders is that there are too many stops, resulting in the line being the slowest branch.

The B line was built in three sections. Kenmore Square to Packards Corner was part of a line to Union Square, while the part from Chestnut Hill Ave to Boston College was an extension of the Beacon St line. Both segments were built sometime between 1895 and 1898. By 1904 the section in between had been built. At Boston College the line once connected with the Commonwealth Avenue Street Railway, later a branch of the Middlesex & Boston Street Railway, hence why the reservation continues down most of Comm Ave almost to Rt. 128. There have never been any plans to restore trolley service west of Boston College, however.