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.

The Los Angeles Metro Rail is an urban rail transit system serving Los Angeles County, California in the United States. It consists of six lines: four light rail lines and two rapid transit lines, serving a total of 101 stations. It connects with the Metro Busway bus rapid transit system, the Metrolink commuter rail system, as well as several Amtrak lines. Metro Rail is owned and operated by the Los Angeles County Metropolitan Transportation Authority (Metro).