The D Line Subway Extension Project, formerly known as the Westside Subway Extension, the Subway to the Sea, and the Purple Line Extension, is a construction project in Los Angeles County, California, extending the rapid transit D Line of the Los Angeles Metro Rail system from its current terminus at Wilshire/Western in Koreatown, Los Angeles, to the Westside region. The project is being supervised by the Los Angeles County Metropolitan Transportation Authority (Metro). The subway has been given high priority by Metro in its long-range plans, and funding for the project was included in two county sales tax measures, Measure R and Measure M.

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).