The C Line (formerly the Green Line from 1995 to 2020) is a 19.3-mile (31.1 km) light rail line running between Redondo Beach and Norwalk within Los Angeles County, California. It is one of six lines forming the Los Angeles Metro Rail system and opened on August 12, 1995. Along the route, the line serves the cities of Downey, El Segundo, Hawthorne, Norwalk and Lynwood, the Los Angeles community of Westchester, and several unincorporated communities in the South Los Angeles region including Athens, Del Aire, and Willowbrook. A free shuttle bus to Los Angeles International Airport (LAX) is available at the line's Aviation/LAX station.
 The fully grade-separated route (essentially a light metro) runs mainly in the median strip of the Century Freeway (I-105) for its eastern portion and on an elevated viaduct for its western portion.
 A segment connecting to the K Line via a wye is expected to open in 2024; the C and K Lines will be integrated and services realigned at that time. The C Line's western terminus will be redirected to LAX/Metro Transit Center station and the K Line extended to Redondo Beach station. A connection to the new LAX Automated People Mover is planned for late 2024.
 The entire route of the C Line is grade-separated, with its tracks following the median of the Century Freeway (I-105) on an elevated guideway. The line begins in the west at Redondo Beach station, then heads roughly north through El Segundo. At Aviation/LAX station, passengers can transfer to any one of several bus lines from different operators, including LAX Shuttle Route M, which provides free service to Los Angeles International Airport. From here, the C Line heads east in the median of the Century Freeway, with a connection to the J Line bus rapid transit line at Harbor Freeway station. It then continues to a major transfer connection at Willowbrook/Rosa Parks station (transfer point to the light rail A Line). Finally, the line terminates in Norwalk, at Norwalk station, just east of the San Gabriel River Freeway (I-605). A non-revenue connector at Willowbrook/Rosa Parks station allows trains to transfer to A Line tracks for maintenance and other non-revenue operations.
 The line does not serve Downtown Los Angeles. Still, passengers can reach it by connecting with the bus rapid transit J Line at Harbor Freeway station, the light rail A Line at Willowbrook/Rosa Parks station, or Metro Bus Express route 460 at Norwalk station.
 C Line service hours are approximately from 4:00 a.m. until 12:30 a.m. daily. Trains operate every 10 minutes throughout the day. Night and early morning weekend service is every 20 minutes.  
 The C Line is the fastest light rail line in the Los Angeles Metro Rail network because trains can operate at speeds up to 65 miles per hour (105 km/h) for most of their route as trains run in the median of the I-105 freeway, not having at-grade street service like other lines such as the A Line. The line has complete grade separation, relatively long station spacing, and a primarily straight alignment.
 The C Line takes 34 minutes to travel 19.5 miles (31.4 km), at an average speed of 34.4 miles per hour (55.4 km/h). This is 43% faster than the A Line, and 81% faster than the E Line.
 The C Line consists of the following 14 stations (from west to east):
 In 1972, Caltrans signed a consent decree to allow construction of the fiercely opposed Century Freeway (Interstate 105), which included provisions for a transit corridor in the freeway's median as a way to help communities impacted by the new freeway.
 Construction began in 1987 on the corridor as a light rail line, envisioned as a connection with the bedroom communities in the Gateway Cities along the Century Freeway with the then-burgeoning aerospace center in El Segundo. The section in El Segundo would be fully elevated and follow the route of the Harbor Subdivision.
 From the beginning of the project, several compromises were made. Because Caltrans dropped a plan for the freeway to cross through Norwalk to Interstate 5, the line was denied a connection to the then-new Norwalk/Santa Fe Springs Metrolink station. Additionally, although planners planned to add a spur to LAX, they did not include it in the initial project over fears that commuters would not use the line if they had to go through the airport on the way to work. The proposed extension to LAX was further complicated by concerns from the Federal Aviation Administration that the overhead lines of the rail line would interfere with the landing paths of airplanes. Amid ambivalence at LAX and L.A. City Hall, the plans to extend the line to the airport were shelved.
 The line opened on August 12, 1995, more than a year late and $950 million over budget. By that time, the Cold War was over, and the aerospace sector in El Segundo was hemorrhaging jobs. The collapse of jobs in the area and the compromises made during construction limited the line's utility, earning it the nickname "the train to nowhere."
 When the C Line began service, it operated with only one-car trains. However, since its opening, ridership continued growing steadily, peaking at nearly 13 million riders in 2014, prompting Metro to operate two-car trains. The increase was driven by the 5,100 park-and-ride spaces and slowing traffic on the 105 freeway. Ridership on the C Line has not been as high as the A Line, although it did have higher ridership than the L Line (then known as the Gold Line) until 2013. Regardless, Metro can only operate two-car trains on the C Line, since the five Metro-built stations west of the freeway only have room for two-car trains. 
 One of the lessons learned from the line, and the Harbor Transitway built at the same time, was that freeway median stations offer a poor rider experience, requiring customers to descend from bridges or climb stairs from dimly lit underpasses to isolated stations in the middle of a noisy and exhaust-ridden freeway. While stations generally have elevators as a necessary accessibility accommodation, these sometimes fail, and have been known for having sanitation issues; escalators are also often out for maintenance or, with the C Line in particular, only available downward.  
 Beginning on August 12, 2023, Metro began a multi-phased project to gradually replace the aging overhead lines on the almost 30-year-old C Line. This will involve closures on segments of the line at different time periods. The first of these closures ran from August 12 to August 19, between Redondo Beach and Hawthorne/Lennox stations, and August 20 to September 24 between Redondo Beach and Aviation/LAX stations. Service was replaced in the meantime by temporary bus shuttles. 
 Varying service patterns have been proposed for integrating the completed K Line into the rest of the system throughout its planning and construction, all of which have involved sharing trackage and infrastructure facilities with the existing C Line. Although some early proposals would've sent trains through all three directions of the wye that will connect the existing C Line with the new segment, this was rejected by Metro because it would cause too much wear and tear on the track switch mechanisms.
 The debate over service patterns proved somewhat contentious, as the final pattern must balance the needs of riders, operational needs, and the political constituencies of Metro's board members. In 2018, with the line then scheduled to open within the year, the Metro Board of Directors overrode a recommendation by operations staff that would've had a single line operating between Expo/Crenshaw and Norwalk station. Passengers from the Redondo Beach area would have been served by a shuttle to the LAX area, where they would need to transfer to another train to continue east or north. Instead, board members approved a one-year pilot of a configuration that would combine an Expo-to-Norwalk line with another line that would connect Redondo Beach with Willowbrook/Rosa Parks station, allowing transfers to the A and J Lines.  The approved plan would incur higher operating expenses, but board members argued it would retain better transfer opportunities for South Bay residents.
 Ongoing construction delays led to a reassessment of that plan in 2022. Metro recommended public outreach aimed at reformulating the operating plan before the connection to the C Line opens in 2023; in March 2023, Metro indicated that it would recommend Option 2 in the figure above, in which the K Line would run north-south from Expo/Crenshaw to Redondo Beach, and the C Line would run from Norwalk to LAX. On June 22, 2023, Metro's board of directors officially approved the implementation of Option 2 based on staff recommendation and public opinion.
 While most stations on the C Line were built to accommodate three-car trains, the stations built west of the Interstate 105 alignment were only built to accommodate two-car trains. To enable increased capacity of the line, Metro plans to lengthen the platforms at Aviation/Imperial, Mariposa, El Segundo, Douglas, and Redondo Beach. The project would also add traction power substations and replace catenary wire and track ties. In April 2023, the state awarded Metro $95 million for the project, which is expected to cost $141 million. The project is expected to be complete in time for the 2028 Summer Olympics.
 Metro is currently working on the initial environmental study of a corridor extension of the C Line from its Redondo terminus toward the southeast. The C Line Extension would roughly follow the Harbor Subdivision ROW into the South Bay, to the Torrance Regional Transit Center (RTC).
Metro and the public are considering two alternatives in the DEIR: an elevated light-rail extension along Hawthorne Boulevard, and an at-grade extension along a BNSF line beside Condon Avenue. The study of the South Bay Extension will lead to the publication of a Draft Environmental Impact Report (DEIR). The study was expected to be completed in 2011. The project was placed on hold in the spring of 2012 due to uncertain funding. With the passage of Measure M in 2016, $619 million was cited for the Green Line Extension south, and the study resumed. The DEIR was released in January 2023. The study area includes the former Harbor Subdivisions right of way. The extension study includes the Redondo Beach station to the Torrance Transit Center, a 4.5-mile (7.2 km) extension study area.
 According to the LA County Expenditure Plan (Measure M), groundbreaking for the project is scheduled for 2026, with an expected opening in 2030–2033. The timeline is expected to be accelerated under the Twenty-eight by '28 initiative.
 The project will become an extension of the K Line upon the completion of LAX/Metro Transit Center station in late 2024, which will become the C Line's new western terminus and allow K Line trains to terminate at Redondo Beach station.
 The C Line's eastern terminus is 2.8 miles (4.5 km) west of the Norwalk/Santa Fe Springs Metrolink station, which is served by several Metrolink lines and sees heavy use. Norwalk/Santa Fe Springs is also a proposed station on the California High-Speed Rail project. Bus service, primarily via Norwalk Transit line 4, is provided between the Metrolink station and the C Line terminus. Still, schedules are not coordinated with the C Line's arrivals. While plans exist to close the gap, available Measure M funding allows the operation to start in roughly 2052.
 On Metro Rail's internal timetables, the C Line is called line 803. 
 The C Line is operated out of Division 22 (Hawthorne Yard & Shop) and Division 16 (Southwestern Yard). These yards stores the fleet used on the C line. Light maintenance is done on the fleet in Division 22, and heavier maintenance is done in Division 16. Division 22 is located between Redondo Beach and Douglas stations. Trains enter the yard via a junction halfway between the two stations. Norwalk-bound trains (Northbound) may enter, but no exit track exists to continue north. Redondo Beach-bound trains (Southbound) may enter and exit the Yard to continue south. Division 16 is located on the completed section of the K Line near the future site of LAX/Metro Transit Center station in Westchester.
 At the time the Green Line opened, the line used a fleet of Nippon Sharyo P2020 light rail vehicles, which were very similar to the older Nippon Sharyo P865 vehicles used on the Blue Line. In late 2001, the P2020 fleet was transferred to the Blue Line, and the Green Line received new Siemens P2000 railcars that have been operating on the line ever since. Kinki Sharyo P3010 trains are also used. Trains are limited to two-car sets due to platform length limitations at some stations along the line.
 Italics denote future stations.
 Other transit in the United States: