Southern Pacific Co. v. Arizona, 325 U.S. 761; 65 S. Ct. 1515; 89 L. Ed. 1915 (1945)

Facts—The Arizona Train Limit Law required that any person or corporation operating within the state a railroad train with more than fourteen passenger cars or more than seventy freight cars would pay a penalty for each violation of the act.

Question—Does the state statute regulating the length of trains contravene the commerce clause of the federal Constitution?

Decision—Yes.

Reasons—C.J. Stone (7–2). The Court reasoned that the Arizona law, viewed as a safety measure, afforded at most slight and dubious advantage, if any, over unregulated train lengths, because it resulted in an increase in expense and in the number of trains and train operations and a consequent increase in train accidents of a character generally more severe than those due to slack action in long trains. Its effect on commerce was regulation without securing uniformity of the length of trains operated in interstate commerce. Thus it prevented the free flow of commerce by delaying it and by substantially increasing its cost and impairing its efficiency.

In dissent, J. Black argued that in the absence of congressional legislation regulating the subject, the Supreme Court should not second-guess state legislative judgments related to matters of safety.