The Denver & Rio Grande Railway (D&RG) was incorporated on October 27, 1870 by General William Jackson Palmer and a board of four directors. It was originally announced that the new 3 ft railroad would proceed south from Denver and travel an estimated 875 miles 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 purist 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 Green River, Utah 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.

The D&RG built west from Pueblo reaching Cañon City in 1874. The line through the Royal Gorge reached Salida on May 20, 1880 and was pushed to Leadville later that same year.

From Salida, the D&RG pushed west over the Continental Divide at the 10,845 feet Marshall Pass and reached Gunnison on August 6, 1881. From Gunnison the line entered the Black Canyon of the Gunnison River passing the famous Curecanti Needle seen in their famous Scenic Line of the World Herald. The tracks left the increasingly difficult canyon at Cimmaron and passed over Cerro Summit, reaching Montrose on September 8, 1882. From Montrose, a line was laid north through Delta, reaching Grand Junction in March 1883, which completed a narrow-gauge transcontinental link with the Rio Grande Western Railway to Salt Lake City, Utah.

The line from Pueblo to Leadville was upgraded in 1887 to three rails to accommodate both narrow-gauge and standard-gauge operation. Narrow-gauge branch lines were constructed to Crested Butte, Lake City, Ouray and Somerset.

The route over Tennessee Pass was known for steep grades, and it was not uncommon to see trains running with midtrain and rear-end helpers. In 1997, a year after the D&RGW/SP merger with the Union Pacific, the UP closed the line. Although it has been out of service for nearly 2 decades, the rails are still in usable condition, though many of the signals have been ravaged by time and vandals.

In 2011, under a federal Beautification Grant, the railroad's overhead signal pole lines were removed, for scrap by a private contractor.

The D&RG also pushed west from Walsenburg, Colorado over La Veta Pass (now "Old La Veta Pass") by 1877. At the time the 'Uptop' depot on Veta Pass, rising over 9,500 feet (2,900 m) in elevation, boasted the highest elevation for a narrow-gauge railroad. The railroad reached Alamosa by 1878. From Alamosa, a line was pushed south through Antonito eventually reaching Santa Fe, New Mexico (the Chili Line) and west as far as Creede, Colorado.

A line containing one of the longest tangent tracks in U.S. railroading (52.82 miles) also linked Alamosa with Salida to the north. From Antonito a line was built over 10,015 feet Cumbres Pass, along the Colorado-New Mexico border, reaching Durango, Colorado in August 1881 and continuing north to the rich mining areas around Silverton in July 1882. A line was also constructed in 1902 as a standard-gauge line, perhaps in anticipation of possible standard gauging of the entire line, south from Durango, Colorado to Farmington, New Mexico. Originally hauling mainly agricultural products and serving as a deterrent to the Santa Fe building up from the south, the line was converted to narrow gauge in 1923, and later delivered pipe and other construction materials to the local oil and natural gas industry into the 1960s.

Portions of the Alamosa-Durango Line survive to this day. The Walsenburg-Alamosa-Antonito line survives as the standard-gauge San Luis and Rio Grande Railroad, with passenger excursion trains service provided by the Rio Grande Scenic Railroad. Two narrow-gauge segments survive as steam railroads, the Antonito-Chama line as the Cumbres and Toltec Scenic Railroad and Durango-Silverton as the Durango and Silverton Narrow Gauge Railroad.

The D&RG built west from Leadville over 10,240 feet Tennessee Pass in an attempt to reach the mining areas around Aspen, Colorado before its rival railroad in the area, the Colorado Midland, could build a line reaching there.

The D&RG built a line through Glenwood Canyon to Glenwood Springs, reaching Aspen in October 1887. The D&RG then joined with the Colorado Midland to build a line from Glenwood Springs connecting with D&RG at Grand Junction. Originally considered a secondary branch route to Grand Junction, the entire route from Leadville to Grand Junction was upgraded to standard gauge in 1890, and the original narrow-gauge route via Marshall Pass became a secondary route.

The original Denver & Rio Grande Western Railway built a narrow-gauge line from Ogden, Utah via Soldier Summit, Utah to Grand Junction, Colorado.

The railroad became the Rio Grande Western Railway in 1889 as part of a finance plan to upgrade the line from narrow gauge to standard gauge, and built several branch lines in Utah to reach lucrative coal fields. It was the railway which Gustaf Nordenskiöld employed to haul boxcars of relics from the Mesa Verde, Colorado, cliff dwellings, in 1891, en route to the National Museum of Finland. In 1901, the Denver & Rio Grande merged with the Rio Grande Western, consolidating in 1908. However, the railroad was weakened by speculators, who had used the Rio Grande's equity to finance Western Pacific Railroad construction. The United States Railroad Administration (USRA) took over the D&RG during World War I.

In 1918 the D&RG fell into receivership after the bankruptcy of Western Pacific. The Denver & Rio Grande Western Railroad (D&RGW or DRGW) was incorporated in 1920, and formally emerged as the new re-organization of the old Denver & Rio Grande Railroad on July 31, 1921.

In 1931, the D&RGW acquired the Denver and Salt Lake Western Railroad, a paper railroad subsidiary of the Denver and Salt Lake Railroad, (D&SL) which had acquired the rights to build a 40-mile connection between the two railroads. After years of negotiation, the D&RGW gained trackage rights on the D&SL from Denver to the new cutoff.

In 1932, the D&RGW began construction of the Dotsero Cutoff east of Glenwood Springs to near Bond on the Colorado River, at a location called Orestod (Dotsero spelled backward). Construction was completed in 1934, giving Denver a direct transcontinental link to the west. The D&RGW slipped into bankruptcy again in 1935. Emerging in 1947, it merged with the D&SL on March 3, 1947, gaining control of the "Moffat Road" through the Moffat Tunnel and a branch line from Bond to Craig, Colorado.

Finally free from financial problems, the D&RGW now possessed a direct route from Denver to Salt Lake City (the detour south through Pueblo and Tennessee Pass was no longer required for direct service), but a problem still remained: for transcontinental service, the Union Pacific's more northerly line was far less mountainous (and, as a result, several hours faster). The D&RGW's solution was its "fast freight" philosophy, which employed multiple diesel locomotives pulling short, frequent trains. This philosophy helps to explain why the D&RGW, despite its proximity to one of the nation's most productive coal mining regions, retired coal-fueled steam locomotives as quickly as new, replacement diesels could be purchased. By 1956, the D&RGW's standard-gauge steam locomotives had been retired and scrapped.

The reason for this was that unlike steam locomotives, diesel locomotives could easily be combined, using the diesels' multiple unit (MU) capabilities, to equip each train with the optimum horsepower which was needed to meet the D&RGW's aggressive schedule.

The D&RGW's sense of its unique geographical challenge found expression in the form of the California Zephyr, a passenger train which was jointly operated with the Chicago, Burlington and Quincy Railroad (CB&Q) from Chicago to Denver, the D&RGW from Denver to Salt Lake City, and the Western Pacific Railroad from Salt Lake City to Oakland, California (with ferry and bus connections to San Francisco).

Unable to compete with the Union Pacific's faster, less mountainous route and 39-hour schedules, the California Zephyr offered a more leisurely journey – a "rail cruise" – with ample vistas of the Rockies. Although the California Zephyr ran at full capacity and turned a modest profit from its 1949 inception through the late 1950s, by the mid-1960s the train was profitable only during the late spring, summer, and fall. In 1970, Western Pacific, claiming multimillion-dollar losses, dropped out. However, the D&RGW refused to join the national Amtrak system, and continued to operate its share of the Zephyr equipment as the Rio Grande Zephyr between Denver and Salt Lake City until 1983, when Amtrak rerouted the San Francisco Zephyr to the Moffat Road line and renamed it the California Zephyr.

Even as the D&RGW exploited the best new standard-gauge technology to compete with other transcontinental carriers, the railroad continued to operate the surviving steam-powered narrow-gauge lines, including the famed narrow-gauge line between Durango and Silverton, Colorado. Most of the remaining narrow-gauge trackage was abandoned in the 1950s and 1960s. At the end of 1970 it operated 1,903 miles of road on 3,227 miles of track; that year it carried 7,733 ton-miles of revenue freight and 21 million passenger-miles.

Two of the most scenic routes survived in operation by the D&RGW, until they were sold to tourist railroad operators.

The Cumbres and Toltec Scenic Railroad assumed operation of the line between Antonito, Colorado and Chama, New Mexico in 1970. The last D&RGW narrow-gauge line, from Durango to Silverton, was sold in 1981 to the Durango and Silverton Narrow Gauge Railroad, exactly one hundred years after the line went into operation.

In 1988, Rio Grande Industries, the company that controlled the D&RGW under the direction of Philip Anschutz, purchased the Southern Pacific Transportation Company (SP). The D&RGW used Southern Pacific's name with SP due to its name recognition among shippers.

In time, the D&RGW's fast freight philosophy gave way to SP's long-established practice of running long, slow trains. A contributing factor was the rising cost of diesel fuel, a trend that set in after the 1973 oil crisis, which gradually undermined the D&RGW's fuel-consuming "fast freight" philosophy. By the early 1990s, the combined Rio Grande/Southern Pacific system had lost much of the competitive advantage that made it attractive to transcontinental shippers, and became largely dependent on hauling the high-quality coal produced in the mine fields of Colorado and Utah.

D&RGW locomotives retained their reporting marks and colors after the consolidation with the Southern Pacific and would do so until the Union Pacific merger.

The one noticeable change was to Southern Pacific's "Bloody Nose" paint scheme. The serif font on the sides of the locomotives was replaced by the Rio Grande's "speed lettering", which was utilized on all SP locomotives built after the merger.

On September 11, 1996, Anschutz sold the combined D&RGW/SP system with the parent company Southern Pacific Rail Corporation to the Union Pacific Corporation, partly in response to the earlier merger of Burlington Northern and Santa Fe which formed the Burlington Northern and Santa Fe Railway.

As the Union Pacific absorbed the D&RGW into its system, signs of the fabled mountain railroad's existence are slowly fading away. D&RGW 5371, the only original D&RGW locomotive in full Rio Grande paint on the Union Pacific, was retired by UP in December, 2008. As previously promised by UP, the D&RGW 5371 was donated to the Utah State Railroad Museum at Ogden's Union Station on August 17, 2009, and will reside in the Eccles Rail Center at the south end of the building. The museum is located at 25th Street and Wall Ave in Ogden, Utah. Many other Rio Grande locomotives still run in service with Union Pacific, but have been "patch-renumbered," with a patch applied over the locomotive's number and the number boards replaced.

This method allows the locomotives to be numbered into the Union Pacific's roster but is cheaper than fully repainting the engine into UP Armour Yellow.

In 2006, Union Pacific unveiled UP 1989, an EMD SD70ACe painted in a stylized version of the DRGW color scheme. This unit is one of several SD70ACe locomotives the UP has painted in stylized colors to help preserve the image of the railroads it has merged; the others are Missouri Pacific Railroad, Missouri-Kansas-Texas Railroad, Chicago and North Western Railway, Southern Pacific Railroad, and Western Pacific Railroad.