By the late 16th century, Spanish exploration and colonization had advanced from Mexico City northward by the great central plateau to its ultimate goal in Santa Fe. Until Mexican independence in 1821, all communications between New Mexico and the rest of the world were restricted to this trail. Over it came ox carts and mule trains, missionaries and governors, soldiers and colonists. When the Santa Fe Trail was established as an overland route between Santa Fe and Missouri, traders from the United States extended their operations southward down the Chihuahua Trail and beyond to Durango and Zacatecas. Ultimately superseded by railroads in the 19th century, the ancient Mexico City–Santa Fe road was revived in the mid-20th century as one of the great automobile highways of Mexico. The part that runs from Santa Fe, New Mexico to El Paso, Texas, US State Highway 85, was pioneered by Franciscan missionaries in 1581 and may be the oldest highway in the United States.