The Sunset Route is a main line of the Union Pacific Railroad running between Southern California and New Orleans, Louisiana. It is the southernmost railway that connects the central United States to the U.S. Pacific Coast.

The idea for this railroad dated before the American Civil War, as businessmen in the Southern United States wanted a direct connection to the Pacific Ocean. This was the rationale for the Gadsden Purchase.

The name traces its origins to the Galveston, Harrisburg and San Antonio Railway, a Southern Pacific Railroad subsidiary which was known as the Sunset Route as early as 1874.^{[citation needed]} The line was built by several different companies and largely consolidated under Southern Pacific, with completion at the Colorado River in 1883. Its construction prompted a frog war at the Colton Crossing, where it intersects the Southern Transcon, then owned by the Atchison, Topeka and Santa Fe Railway, and now by BNSF Railway.

The first trains departed for through service between Los Angeles, California and New Orleans on February 5, 1883.

The Sunset Route previously extended northward from Los Angeles to San Francisco, California.

The Sunset Route's western terminus officially begins about 71 miles (115 km) east of Los Angeles, in West Colton, California. Going eastward, the route immediately faces steep inclines of up to 1.9% as it reaches 2,560 feet (780 m) in elevation at Beaumont Hill, near Palm Springs, California. It then turns southeast, quickly dropping to 201 feet (61 m) below sea level at Wister, California on the landlocked Salton Sea. The route rises to 385 feet (117 m) in elevation before dropping again as it goes through Yuma, Arizona, near the California and Mexico borders and close to where the Gila River discharges into the Colorado River. It turns directly east to Maricopa, Arizona, at the southern edge of the Phoenix metropolitan area, before turning to the southeast again to Tucson, Arizona. Heading eastward again, the route rises to 4,557 feet (1,389 m) at Dragoon in southeast Arizona, drops to 3,600 feet (1,100 m) in San Simon, Arizona, and then crosses the Continental Divide at 4,554 feet (1,388 m) elevation at Wilna in southwest New Mexico before crossing into Texas at the Mexican border metropolis of El Paso. The Sunset Route is the lowest railway that crosses the Continental Divide.

At El Paso, the Sunset Route splits off into the Golden State Route, which is another main line that Union Pacific acquired in the Southern Pacific merger that heads northeast to Kansas City, Missouri and Chicago, Illinois. The Sunset Route itself turns southeast past El Paso near the Rio Grande River within the Trans-Pecos region of West Texas. At the town of Sierra Blanca, Texas, 90 miles (140 km) from El Paso, the Sunset Route meets the western end of the former Texas and Pacific main line, acquired by Union Pacific when it merged with Missouri Pacific, that goes east-northeast through the Dallas-Fort Worth metroplex. The Sunset Route continues heading roughly southeast within the Rio Grande watershed until it arrives at the Mexican border city of Del Rio, Texas; at that point, the route turns directly east through San Antonio, Texas, the Gulf Coast metropolis of Houston, Texas, and the eastern terminus of New Orleans, in southeast Louisiana on the Mississippi River Delta. Between San Antonio and New Orleans, the Sunset Route meets several other Union Pacific-controlled main lines and branch lines, of which many are directional running operations.

BNSF shares ownership of the Lafayette Subdivision.