Richmond–Millbrae+SFO line

When the SFO/Millbrae extension opened on June 22, 2003, the Richmond–Daly City/Millbrae line continued to terminate at Daly City. BART extended this line to San Francisco International Airport and Millbrae during weekday peak hours on February 9, 2004. San Mateo County is not a member of the San Francisco Bay Area Rapid Transit District, so SamTrans funded the county's BART service. When the extension's lower-than-expected ridership caused SamTrans to accrue deficits, BART agreed to SamTrans' request to operate only the Dublin/Pleasanton line south of Daly City effective September 12, 2005.

SamTrans and BART reached an agreement in February 2007 in which SamTrans would transfer control and financial responsibility of the SFO/Millbrae extension to BART, in return for BART receiving additional fixed funding from SamTrans and other sources.[8]

Beginning March 22, 2021, Richmond–Millbrae line trains were interlined with the SFO-Millbrae line, while Saturday service was discontinued.[9] On August 2, 2021, the line began operating on weekdays and Saturdays until 9 pm, with all trains extended to SFO as the Richmond–Millbrae + SFO line.[10] On February 20, 2022, the line began operating on all days until 9 pm, except on some Sundays when power cable replacement work takes place in San Francisco.[11]

On March 6, 2022, a break in a power cable near Berkeley caused SFO–Richmond service to be temporarily discontinued. On March 8, a shuttle service began operating between SFO and Millbrae.[12] Red Line service resumed with 5-car trains on March 22.[13]