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.

[2] 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.

[5][6] The Sunset Route's western terminus officially begins about 71 miles (115 km) east of Los Angeles, in West Colton, California.

The Sunset Route itself turns southeast past El Paso near the Rio Grande River within the Trans-Pecos region of West Texas.

[21] The Amtrak Sunset Limited operates three round-trips weekly over the entirety of the route, and the Texas Eagle from Chicago is attached between San Antonio and Los Angeles.

In December 2023, the Federal Railroad Administration (FRA) announced a grant through its Corridor ID Program to increase the frequency of the Sunset Limited to full daily round-trip service.

[7][24] However, the combined company's efforts to expand double-trackage between Los Angeles and El Paso were soon delayed in favor of more profitable investments on Union Pacific's pre-existing lines north of the Sunset Route to improve coal-hauling capacity or to better handle mixed-freight trains along the Central Corridor.

Both lines funneled additional trains onto the Sunset Route's Los Angeles–El Paso section, which exacerbated the traffic meltdown that occurred there in 2003–2004 after Union Pacific underestimated the strong economic recovery from the early 2000s recession.

[10] Work to add the second track picked up in the mid-2000s,[19][25] and by late 2007, Union Pacific was targeting the complete double-tracking of the 757-mile (1,218 km) Los Angeles–El Paso section by the end of 2010.