The Flagstaff Railroad Addition Historic District is significant because of its association with the Atchison, Topeka and Santa Fe Railway as well as U.S. Route 66. The original boundary was roughly bounded by Santa Fe RR tracks, Agassiz and Beaver Sts., Birch and Aspen Avenues. The district was expanded twice to add nine buildings along Phoenix Avenue from Beaver Street to San Francisco Avenue, and a building at 122 East Route 66.