The Los Angeles Metro Rail Gold Line was L.A.'s third light rail line to open and is completely isolated from the other three although their rolling stock is completely interchangeable, and like all L.A. Metro Light Rail lines it has only high-level platforms. The 13 mile northern half of the line opened on from Union Station to Sierra Madre Villa opened on July 26, 2003, it mostly follows the historical ROW out of Los Angeles to Pasadena formerly used by the ATSF Railway, with the last Amtrak Southwest Chief to run in January 1994. The six mile southern extension which includes a 1.8 mile tunnel, and street running everywhere else, officially named the Edward R. Roybal Linea de Oro Eastside Extension, opened on November 15, 2009. The 11.5 mile Foothill Extension from Sierra Madre Villa to Azusa opened on March 5, 2016. Extending the line a further 12.6 miles to Montclair is in the advanced stages of the environmental process, and extremely early initial studies have been made that could eventually bring the Gold Line out of Los Angeles County, into San Bernardino County to Ontario Airport. The line was originally envisioned as an extension of the blue line which terminates at 7th/Metro Center, with the Red Line already built through downtown this was seen as an expensive unnecessary project. Today though a 1.9 mile tunnel called the Regional Connector with two intermediate stations is in the advanced stages of the environmental process, mostly funded and due to open by 2022. It will completely change the operations of light rail in Los Angeles and discontinue today's Gold Line by through routing the Blue Line to Pasadena and onto the Foothill extension (a north-south route) and the Expo Line out to East L.A.

The Gold Line is the slowest Metro Rail line by average speed and has the lowest ridership of the four rail lines (43,136 daily boardings in March 2012 and from February 2006 to December 2007 six express trains per rush hour per direction ran every half-hour stopping only at Sierra Madre Villa, Del Mar, Mission, Highland Park, and Union Station, local trains ran every 15 minutes then. Today the rush hour frequency is every 6 minutes, with 12 during middays, and 20 during evenings. Weekend service is every 15 minutes.