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).

Los Angeles County, officially the County of Los Angeles, and sometimes abbreviated as L.A. County, is the most populous county in the United States, with 9,861,224 residents estimated in 2022. Its population is greater than that of 40 individual U.S. states. Comprising 88 incorporated cities and many unincorporated areas within a total area of 4,083 square miles (10,570 km2), it is home to more than a quarter of Californians and is one of the most ethnically diverse U.S. counties. The county's seat, Los Angeles, is the second most populous city in the United States and the most populous city in California, with 3,822,238 residents estimated in 2022. It is also known for being the home of the American film and television industry, a distinction it has held since the early 20th century which has given the county global prominence.