Coaster (stylized in all caps) (reporting marks NCTC, SDNX) is a commuter rail service in the central and northern coastal regions of San Diego County, California, United States, operated by the North County Transit District (NCTD).

The 41-mile (66 km) commuter rail line features eight stops, with a travel time of about one hour and one minute end-to-end. The service operates seven days a week except nights, with slightly greater frequency during weekday peak periods.

Coaster first entered service on February 27, 1995, and has since grown in ridership and capacity. In 2024, the line had a ridership of 871,200, or about 3,200 per weekday as of the second quarter of 2025.

The North San Diego County Transit Development Board was created in 1975 to consolidate and improve transit in northern San Diego County. Planning began for a San Diego–Oceanside commuter rail line, then called Coast Express Rail, in 1982. Funding for right-of-way acquisition and construction costs came from TransNet, a 1987 measure that imposed a 0.5% sales tax on San Diego County residents for transportation projects. The Board established the San Diego Northern Railway Corporation (SDNR) – a nonprofit operating subsidiary – in 1994. SDNR purchased the 41 miles (66 km) of the Surf Line within San Diego County plus the 22-mile (35 km) Escondido Subdivision (later used for the SPRINTER) from the Santa Fe Railway that year.

Coaster service began on February 27, 1995. NCTD originally contracted Amtrak to provide personnel for Coaster trains. In July 2006, TransitAmerica Services took over the day-to-day operation of the commuter train, based on a five-year, $45 million contract with NCTD. In 2016, Bombardier Transportation replaced TransitAmerica as Coaster's operator. In December 2018, NCTD achieved full implementation of positive train control along the entire Coaster route, making it one of only four railroads in the United States to achieve full implementation of this technology without needing an extension beyond 2018.

San Diego County voters extended the TransNet sales tax through 2038, which includes funding for rail track upgrades. By the early 2010s, numerous improvements such as added double track and bridge replacements were in various stages of construction and design. As part of the broader North Coast Corridor project, approximately $1 billion is planned to be spent on new segments of double track between San Diego and Orange County.

Limited-use stations at the San Diego Convention Center and the Del Mar Racetrack for use during major events have been planned, and is expected to complete construction of the stations in late 2025. A northward extension to Camp Pendleton was also proposed in 2011.

In the early 2020s, officials recognized the need to move the tracks away from the encroaching erosion of the Del Mar bluffs. Planning and design of a rail tunnel under the city are ongoing, with an estimated completion date of 2035. By 2024, SANDAG narrowed down the Del Mar bluffs bypass to three routes. One of the routes would tunnel under the Del Mar Fairgrounds, which was opposed by the local community.