This primary source comes from the Records of the Office of the Chief of Engineers.
Full Citation: Map of the Railroad Survey from Omaha, Nebraska, to San Francisco, California; 6/30/1867; New Mexico, Utah, California, Oregon, and Washington; Civil Works Map File, 1818 - 1947; Records of the Office of the Chief of Engineers, Record Group 77; National Archives at College Park, College Park, MD. [Online Version, https://www.docsteach.org/documents/document/map-survey-transcontinental-railroad, April 26, 2024]