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.

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.

[4] 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.

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.

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 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.

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.

The line continued building west until reaching the D&RGW close to present day Green River which completed a narrow-gauge transcontinental link with the Rio Grande Western Railway to Salt Lake City, Utah.

Narrow-gauge branch lines were constructed to Chama, New Mexico, Durango, Silverton, Crested Butte, Lake City, Ouray and Somerset, Colorado.

From Antonito a line was built over 10,015 feet (3,053 m) 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.

Part of the reason for this isolated change of gauge was that the Southern Pacific contemplated extending to access coal fields in the northern San Juan basin, had surveyors working there, and had incorporated a subsidiary, the Arizona & Colorado Railroad Company, for this purpose.

[6] Originally hauling mainly agricultural products, the Farmington line was converted to narrow gauge in 1923,[7] and later delivered pipe and other construction materials to the local oil and natural gas industry into the 1960s.

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.

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).

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.

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 3/4-hour schedules, the California Zephyr offered a more leisurely journey – a "rail cruise" – with ample vistas of the Rockies.

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 5371 was donated to the Utah State Railroad Museum at Ogden's Union Station on August 17, 2009, and resides in the Eccles Rail Center at the south end of the building.

This unit is one of several SD70ACe locomotives in the Union Pacific heritage fleet has painted in stylized colors to help preserve the image of the railroads it has merged.