The C Line (formerly the Green Line from 1995 to 2020) is a 19.3-mile (31.1 km) light rail line running between the Los Angeles neighborhood of Westchester and the city of 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 also serves the cities of Downey, Hawthorne, and Lynwood, as well as several unincorporated communities in the South Los Angeles region including Athens, Del Aire, and Willowbrook. The fully grade-separated route (essentially a light metro) runs mainly in the median strip of Interstate 105 (the Century Freeway) for its latitude portion and in a mixture of viaducts, embankments, and an open trench for its western leg. A free shuttle bus to Los Angeles International Airport (LAX) is available at Aviation/LAX station.

On November 3, 2024, the C and K lines underwent service changes to accommodate pre-revenue testing at the upcoming LAX/Metro Transit Center station, scheduled to open in 2025. The C Line's western terminus was redirected to Aviation/Century station and the southern segment of the K Line was created between Aviation/Century station and Redondo Beach station. Once LAX/Metro Transit Center station opens, the C Line will terminate at this station, and the K Line will operate as a single, continuous service through it. The LAX Automated People Mover, which will offer a connection to the airport's terminals, will begin service at LAX/Metro Transit Center station in 2026.

The entire route of the C Line is grade-separated, with its tracks following the median of Interstate 105 (the Century Freeway) and on an elevated guideway. The line begins at Aviation/Century station, situated on a viaduct above Century Boulevard and shared with the K Line. The line will then briefly enter an open trench as it passes close to the LAX runways before splitting from each other at a wye west of Aviation/LAX station. 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.

Passengers can reach Downtown Los Angeles 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. Metrolink service to Downtown Los Angeles via Union Station, Perris, and Oceanside, as well as other cities in Orange County and Riverside County, can be accessed from the C by transferring to Norwalk Transit route 4 and disembarking at Norwalk/Santa Fe Springs station.

On November 3, 2024, the C Line was redirected to begin at Aviation/Century station from its prior terminus at Redondo Beach station. The K Line subsumed the westernmost segment of the C Line west of the wye, running south before ending at Redondo Beach station. Until LAX/Metro Transit Center station opens, Metro is operating a bus shuttle called the "C & K Line Link" from Westchester/Veterans station to Aviation/LAX station via Aviation/Century 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.

Time	4-7A	8A-6P	7P	8-12A
Weekdays	10		10-20	20
Weekends/Holidays	20	10	10-20	20

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 11 stations (from west to east):

Station	Date Opened	City/Neighborhood	Major connections and notes
LAX/Metro Transit Center	2025	Los Angeles (Westchester)	LAX Automated People Mover (2026)
Aviation/​Century	November 3, 2024	Los Angeles (Westchester)	(southern segment) LAX via LAX Shuttle
Aviation/LAX	August 12, 1995		LAX via LAX Shuttle  (northern segment) via C & K Line Link Park and ride: 435 spaces
Hawthorne/​Lennox		Hawthorne	SoFi Stadium via shuttle bus Park and ride: 362 spaces
Crenshaw			Park and ride: 506 spaces
Vermont/Athens		Athens	Park and ride: 155 spaces
Harbor Freeway		South Los Angeles	Park and ride: 253 spaces
Avalon			Park and ride: 160 spaces
Willowbrook/​Rosa Parks	July 14, 1990	Willowbrook	Park and ride: 234 spaces
Long Beach Boulevard	August 12, 1995	Lynwood	Park and ride: 635 spaces
Lakewood Boulevard		Downey	Park and ride: 403 spaces
Norwalk		Norwalk	Park and ride: 1,759 spaces

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.

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.

A new infill station is planned to be constructed between Long Beach Boulevard and Lakewood Boulevard in order to provide an interchange with the forthcoming Southeast Gateway Line. It is expected to open with the new service in 2035.

On Metro Rail's internal timetables, the C Line is numbered 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.

As of 2024, the Kinki Sharyo P3010 is the only rolling stock to serve the C Line.

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 (now known as A 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, until its removal in 2024 and subsequent transfer to the A Line. Since 2019, newer Kinki Sharyo P3010 trains were added to the Green Line alongside P2000s to supplement the fleet and have been used since then. Trains are limited to two-car sets due to platform length limitations at some stations along the line. Trains are limited to two-car sets due to platform length limitations at some stations along the line.

• On February 22, 2015, a train near the Hawthorne/Lennox station struck and killed a man who was trespassing onto the tracks.
• On August 24, 2018, a collision involving a tanker truck on the westbound lanes of the I-105 between Vermont Avenue and Crenshaw Boulevard exploded with flames that crawled onto the tracks and damaged the catenary system. The rail line and the freeway were closed for cleanup and repairs.

• MTA Home Page Archived November 11, 2020, at the Wayback Machine
• Green Line Home Page Archived January 25, 2010, at the Wayback Machine
• Green Line connections overview Archived June 26, 2013, at the Wayback Machine
• Green Line schedule