Alexander Gardner became a chief photographer in 1867 for the Eastern Division of the Union Pacific Railway (which changed its name to Kansas Pacific Railway in March 1869).
Gardner, an immigrant from Scotland best known for his photographs of the US Civil War, traveled to St. Louis in 1867, where he met with surveyors searching for the best route for the proposed southern branch of the transcontinental railroad. The survey team traveled from St. Louis to San Francisco via Kansas, Colorado, New Mexico, and Arizona.
The photographs, taken during the expedition in 1867 and 1868, were published in a portfolio of 127 albumen pictures entitled Across the Continent on the Kansas Pacific Railroad (Route of the 35th Parallel) circa 1869.