 The D&RGW operated the highest mainline rail line in the United States, over the Tennessee Pass in Colorado, and the famed routes through the Moffat Tunnel and the Royal Gorge.At its height in the mid-1880s, the D&RG had the largest narrow-gauge railroad network in North America with 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.