The Foothill Extension Project, formerly known as the Gold Line Foothill Extension Project, is an extension of the Los Angeles Metro Rail A Line light rail line from the former Gold Line terminus in Pasadena, California, at Sierra Madre Villa station, east through the "Foothill Cities" of Los Angeles County. The plan's first stage, Phase 2A, extended the then-Gold Line from Pasadena to Azusa; it opened on March 5, 2016. Phase 2B, which will extend the now A Line a further four stations to Pomona–North station, broke ground in December 2017. A further two–station extension to Montclair Transcenter is planned for completion in 2028, but is currently unfunded.

The A Line is a 48.5-mile (78.1 km) light rail line in Los Angeles County, California. It is one of the six lines of the Los Angeles Metro Rail system, operated by the Los Angeles County Metropolitan Transportation Authority (Metro). The A Line serves 44 stations and runs east-west between Azusa and Pasadena, then north-south between Pasadena and Long Beach. It interlines and shares five stations with the E Line in Downtown Los Angeles. Service operates for approximately 19 hours with headways of up to 8 minutes during peak hours. The A Line is currently the longest light rail line in the world.