The Denver and Rio Grande Western Railroad (reporting mark DRGW), often shortened to Rio Grande, D&RG or D&RGW, formerly the Denver & Rio Grande Railroad, was an American Class I railroad company. The railroad started as a 3 ft (914 mm) narrow-gauge line running south from Denver, Colorado, in 1870. It served mainly as a transcontinental bridge line between Denver and Ogden, Utah. The Rio Grande was also a major origin of coal and mineral traffic.

The Rio Grande was a strong example of mountain railroading, with a motto of Through the Rockies, not around them and later Main line through the Rockies, both referring to the Rocky Mountains.

The D&RGW operated the highest mainline rail line in the United States, over the 10,240 feet (3,120 m) Tennessee Pass in Colorado, and the famed routes through the Moffat Tunnel and the Royal Gorge. At its height, in 1889, the D&RGW had the largest narrow-gauge railroad network in North America with 1,861 miles (2,995 km) of track interconnecting the states of Colorado, New Mexico, and Utah. Known for its independence, the D&RGW operated the Rio Grande Zephyr until its discontinuation in 1983. This was the last private intercity passenger train in the United States until Brightline began service in Florida in 2018.

In 1988, the Rio Grande's parent corporation, Rio Grande Industries, purchased Southern Pacific Transportation Company, and as the result of a merger, the larger Southern Pacific Railroad name was chosen for identity. The Rio Grande operated as a separate division of the Southern Pacific until 1992. Today, most former D&RGW main lines are owned and operated by the Union Pacific Railroad while several branch lines are now operated as heritage railways by various companies.

The Denver & Rio Grande Railway (D&RG) was incorporated on October 27, 1870, by General William Jackson Palmer (1836–1909), and a board of four directors. It was originally announced that the new 3 ft (914 mm) railroad would proceed south from Denver and travel an estimated 875 miles (1,408 km) south to El Paso via Pueblo, westward along the Arkansas River, and continue southward through the San Luis Valley of Colorado toward the Rio Grande. Closely assisted by his friend and new business partner Dr. William Bell, Palmer's new "Baby Road" laid the first rails out of Denver on July 28, 1871, and reached the location of the new town of Colorado Springs (then the Fountain Colony) by October 21. Narrow gauge was chosen in part because construction and equipment costs would be relatively more affordable when weighed against that of the prevailing standard gauge. Palmer's first hand impressions of the Ffestiniog Railway in Wales buoyed his interest in the narrow-gauge concept which would prove to be advantageous while conquering the mountainous regions of the Southwest. Eventually the route of the D&RG would be amended (including a plan to continue south from Pueblo over Raton Pass) and added to as new opportunities and competition challenged the railroad's expanding goals.

Feverish, competitive construction plans provoked the 1877–1880 war over right of way with the Atchison, Topeka and Santa Fe Railway. Both rivals hired gunslingers and bought politicians while courts intervened to bring settlement to the disagreements. One anecdote of the conflict recounts June, 1879, when the Santa Fe defended its roundhouse in Pueblo with Dodge City toughs led by Bat Masterson; on that occasion, D&RG treasurer R. F. Weitbrec paid the defenders to leave. In March, 1880, a Boston Court granted the AT&SF the rights to Raton Pass, while the D&RG paid an exorbitant $1.4 million for the trackage extending through the Arkansas River's Royal Gorge. The D&RG's possession of this route allowed quick access to the booming mining district of Leadville, Colorado. While this "Treaty of Boston" did not exactly favor the purest of original D&RG intentions, the conquering of new mining settlements to the west and the future opportunity to expand into Utah was realized from this settlement.

By late 1880, William Bell had begun to organize railway construction in Utah that would become the Palmer controlled Denver & Rio Grande Western Railway in mid-1881. The intention of the D&RGW (aka the "Western") was to work eastward from Provo to an eventual link with westward bound D&RG in Colorado. This physical connection was realized near the Green River on March 30, 1883, and by May of that year the D&RG formally leased its Utah subsidiary as previously planned. By mid-1883, financial difficulties due to aggressive growth and expenditures led to a shake up among the D&RG board of directors, and General Palmer resigned as president of the D&RG in August, 1883, while retaining that position with the Western. Frederick Lovejoy would soon fill Palmer's vacated seat on the D&RG, the first in a succession of post Palmer presidents that would attempt to direct the railroad through future struggles and successes.

Following bitter conflict with the Rio Grande Western during lease disagreements and continued financial struggles, the D&RG went into receivership in July, 1884, with court-appointed receiver William S. Jackson in control. Eventual foreclosure and sale of the original Denver & Rio Grande Railway resulted within two years, and the new Denver & Rio Grande Railroad took formal control of the property and holdings on July 14, 1886, with Jackson appointed as president. General Palmer would continue as president of the Utah line until retirement (due to company re-organization) in 1901.