Surf Line

Construction of the Surf Line between Los Angeles and San Diego began on October 12, 1880, with the organization of the California Southern Railroad Company. On January 2, 1882, the California Southern commenced passenger and freight service between National City and Fallbrook Junction, just north of Oceanside. From Oceanside the line turned northeast for a winding route through the Temecula Canyon, and was finished on August 21, 1882. The section through Del Mar was built in 1881 but was originally built on an alignment about a block inland from the ocean. It was moved to its current location, along the seaside bluffs, in 1910, because the grade was less steep and there were fewer crossings.

The line became part of the Atchison, Topeka and Santa Fe Railroad's transcontinental rail line in 1885 via an extension of the California Southern from Colton north over the Cajon Pass to Barstow. From 1886 to 1888, the Riverside, Santa Ana and Los Angeles Railway built a branch from Highgrove southwest via Riverside to Santa Ana and from Orange (just north of Santa Ana) northwest to Los Angeles. Also in 1888 the San Bernardino and San Diego Railway completed its line from Oceanside north to Santa Ana, completing what was originally called the Los Angeles-San Diego Short Line. The now-downgraded old route was destroyed by floods in 1891 and the new line, later named the Surf Line, was now the only line to San Diego from the north.

In 1910, the Fullerton and Richfield Railway built a short cutoff of the San Bernardino-Los Angeles route from Atwood west to Fullerton, giving the Surf Line its northern terminus of Fullerton.

For much of the 20th century, the Surf Line (officially, the Fourth District of the Los Angeles Division) was to the Santa Fe what the New York City, Philadelphia corridor was to the Pennsylvania Railroad. Daily traffic could reach a density of ten trains (each way) during the summer months. The route hosted AT&SF San Diegan passenger trains, renamed the Pacific Surfliner by Amtrak in 2000. The Santa Fe installed centralized traffic control in 1943-1944 which increased capacity on the line.

Santa Fe sold the line to local transportation authorities in 1992, with ownership split between the Southern California Regional Rail Authority in Orange County and the San Diego Northern Railway in San Diego County.

The route is the southerly portion of the 351-mile-long (565 km) LOSSAN Rail Corridor between San Luis Obispo and San Diego. Local agencies along the route formed the Los Angeles-San Diego-San Luis Obispo Rail Corridor Agency (LOSSAN) in 1989. Commuter trains began operating in the 1990s, initially as an outgrowth of existing Amtrak services until the establishment of Metrolink and Coaster by the California State Legislature in 1992. Coaster runs within San Diego County, between San Diego and Oceanside, while Metrolink's services operate north of Oceanside. Amtrak's Pacific Surfliner travels throughout the corridor. The San Diego Trolley light rail shares the Surf Line's right of way in San Diego, running adjacent to the heavy rail tracks. For about a mile in Oceanside, the Sprinter service parallels the Surf Line before heading east on the Escondido Subdivision towards Escondido, California.

Freight traffic includes military vehicles and equipment to Camp Pendleton and the Navy ports in San Diego. Due to passenger trains running on a daily schedule, BNSF manifest freight trains run through the Surf Line often at night, which is dubbed as the "Daygo".

The Surf Line is the second busiest rail corridor in the United States, after the Northeast Corridor between Washington D.C. and Boston, as the Surf Line hosts the Pacific Surfliner, the third busiest Amtrak route behind the Acela and Northeast Regional, which run on the Northeast Corridor. As trains on the Surf Line traverse speeds up to 90 mph (140 km/h) on portions of the route in Orange County and San Diego County, there are plans to upgrade tracks to Class 6 trackage, which can run at speeds of 110 mph (180 km/h), when funding is available.

About two-thirds of the 60-mile (97 km) segment from the Orange County line to the Santa Fe Depot in downtown San Diego has been double-tracked. As one of the nation's busiest corridors, local transportation and planning agencies want to complete the entire section. A 2.6-mile (4.2 km) section of double track between Elvira (SR 52) and Morena (Balboa Avenue) was completed in July 2020. The $192 million project, which began in August 2015, completed 14.6 miles (23.5 km) of double track from San Diego northward.

The segment of the LOSSAN Corridor within San Diego County achieved full implementation of positive train control in December 2018, for all passenger and freight trains operating on this segment.

Due to its location along the beaches of Southern California, the line faces persistent issues due to sea level rise and coastal erosion, exacerbated by climate change. The tracks run atop coastal bluffs some 40 feet (12 m) above the beach for 1.7 miles (2.7 km) in Del Mar. Another segment along the San Clemente coast on a low-lying section of track crosses an ancient, recurring landslide. Aggravated by storms and high tides, waves sometimes crash across these rails at high tide that are close to the surf on the narrow beach. The eroding bluffs above the tracks in at least three locations require temporary closure of the tracks until conditions can be stabilized. Extra funding for maintenance of both sections continues to be sought to keep them in operation as long-term solutions are developed.

Del Mar Bluffs

Coastal erosion eats away at the Del Mar bluffs each year and the rate has accelerated due to sea level rise due to climate change. The bluffs erode by about six inches every year. The bluff has had to be shored up to safely run current operations. Steel beams were driven into the beach at the base of the bluff in September 2020 to stabilize the face of the bluff for 20 or 30 years. In August, the California Coastal Commission had emphasized the need to move the railroad tracks inland as they reviewed the emergency permits for the stabilization work. The San Diego Association of Governments (SANDAG) is conducting a $3 million study on relocating the rail line. A tunnel under Del Mar, which would cost more than $3 billion, is under consideration. In 2022, $300 million was included in the state budget for the SANDAG so that the project can compete for federal matching funds. Local leaders, including SANDAG's executive director, showed Secretary of Transportation Pete Buttigieg the coastal erosion at the bluffs in October 2022. Del Mar City Council approved a list of guiding principles for the relocation on November 13, 2023. More than a dozen possible routes for the tunnel have been considered by SANDAG. Residents have expressed concerns about the shortest and fastest routed that would take trains beneath residential properties.

San Clemente beach

With extensive beach erosion, waves crashing over the rails damaged the tracks in San Clemente on September 15, 2021. The segment had to be shut down through October 3 as storms and high tides had aggravated the situation. An extended closure occurred the next year when the segment was closed to passenger traffic on September 29, 2022, due to soil movement; freight traffic continued at lowered speed. Additional rock was added between the beach and the railroad tracks after each incident. The passenger rail traffic stop continued as anchors were being placed into bedrock. The line fully reopened in April 2023 but rail service was halted again two miles to the north. Falling debris did not damage the tracks but ground movement continued from a landslide involving Casa Romantica on the bluff above. Full service resumed in late May but was halted indefinitely again in early June. During the work closures, freight trains are able to resume at a speed of 10 miles per hour (16 km/h) as officials were on site to clear them when it is safe to proceed. Orange County Transportation Authority (OCTA), which owns the section of track, declared an emergency on June 12 to speed up construction of temporary barrier wall at the bottom of the slope. The 250-foot-long (76 m) barrier is 12 feet (3.7 m) high with the piles set roughly 32 feet (9.8 m) into the ground. Service resumed on July 19, 2023. The bluffs can become unstable after a rainstorm soaks the slope. Service was suspended on January 25, 2024, due to a landslide from private property north of the San Clemente Pier. Two damaged sections of the Mariposa Pedestrian Bridge on the slope above the tracks had to be removed. California Transportation Commission initially awarded $2 million to clean up the debris and added $7.2 million in February to help repair the rail line. A barrier wall similar to the one at the Casa Romantica slide will be constructed. Limited service through the landslide area resumed on March 6. Full service then resumed on March 25. OCTA says $7 million is needed to study realignment and other possible solutions to protect 7 miles (11 km) of the line along the shore. The first study, that started in August 2023, will identify the issues that impact the rail corridor, offer solutions to protect it, and develop the cost of the necessary improvements.

Many, but not all of these stations currently operate. Many of these stations no longer exist (e.g. Linda Vista) and new ones have opened (e.g. Sorrento Valley). For a list of stations that currently operate, see the articles for Metrolink's Orange County Line and the Coaster or the templates to the right.