The service is descendant of the foundational Key System streetcar and ferry line that operated prior to the formation of AC Transit.

Business interests controlled by Francis Marion "Borax" Smith obtained a franchise to run streetcars on Shattuck and Adeline streets by June 1902, seeking to directly compete the parallel Southern Pacific Berkeley Branch Railroad.

[1] The line opened for traffic on October 26 the following year complete with a ferry connection to San Francisco.

[3] Key System cars began sharing trackage along Shattuck with the Southern Pacific starting in 1930,[4] though Adeline trains were discontinued past Alcatraz Avenue after March 31, 1933.

[12] In September 1961, service was extended down Solano Avenue for evening and Saturday runs,[13] a move which was further applied to all trips starting in 1970.