In 1912, Arizona passed the Arizona Train Limit Law, which made it unlawful for any person or corporation to operate a train of more than fourteen passenger or seventy freight cars in Arizona, and authorized the state to collect a monetary fee from anyone that violated this provision. Southern Pacific Co. (plaintiff) owned and operated many trains running throughout the United States that were all longer than the limits required by the Arizona law. As such, whenever Southern Pacific ran trains through Arizona, it had to shorten the length of its trains and increase the train frequency to make up for the decrease in carrying capacity. This resulted in large increases in operating costs and decreases in efficiency. Southern Pacific brought suit against Sullivan (defendant), the Attorney General of Arizona, alleging that the Arizona state law was an unconstitutional restriction on interstate commerce. The trial court agreed with Southern Pacific, but the court of appeals reversed. Southern Pacific Co. appealed to the United States Supreme Court.