[1] The Court held that the law imposed a burden far greater than necessary to achieve Arizona's legitimate interest in lowering the rate of train accidents.

This case is part of the Court's so-called negative commerce clause jurisprudence.

State laws that discriminate against interstate commerce in favor of intrastate commerce are per se invalid if enacted for the purpose of economic protectionism, but are valid if enacted for a legitimate health and safety reason so long as no reasonable alternative nondiscriminatory means exists to accomplish that health and safety goal.

[2] State laws that do not discriminate against interstate commerce but that burden interstate commerce are subject to a balancing test, later crystallized by the Court in Pike v. Bruce Church, wherein the state law is invalid if its burden on interstate commerce is not outweighed by the legitimate health and safety benefits that derive from the law.

[3] The Court held that there was insufficient benefit to Arizona's legitimate health and safety interests to justify so great a burden on interstate commerce: the Train Limit Law had ostensibly been passed to decrease the rate of rail accidents that "result from the greater length of trains," but the court noted that if anything the result of the law might have been to increase accidents "resulting from the larger number of trains.