The case of Southern Pacific Company v. State of Arizona in 1918 centered around the issue of whether or not a state could regulate interstate commerce, specifically regarding train lengths. The state law in question limited trains to no more than fourteen passenger cars and seventy freight cars within its borders. Southern Pacific Company, an interstate railroad company, challenged this law arguing that it interfered with their operations across multiple states and thus should be regulated by federal rather than state laws under the Commerce Clause of the U.S Constitution. The Supreme Court ruled in favor of Southern Pacific Company stating that while states do have some power to regulate local aspects related to safety concerns, they cannot interfere with interstate commerce when there is a national standard set by Congress or when such regulation places an undue burden on said commerce. In this case, Arizona's law was found to place substantial operational burdens on railroads engaged in interstate business without providing significant safety benefits hence unconstitutional.

The dissenting opinion in the Southern Pacific Company v. State of Arizona case argued that the majority's decision was a misinterpretation of the Commerce Clause, which grants Congress power to regulate interstate commerce. The dissenters believed that this clause did not give federal courts authority to strike down state laws unless they directly conflicted with federal legislation or were clearly an unreasonable burden on interstate commerce. They contended that Arizona's train length law was a reasonable safety measure and did not significantly impede interstate trade, as it only required railroads to use more, shorter trains within its borders rather than fewer, longer ones. Furthermore, they pointed out inconsistencies in how similar cases had been decided previously by the court and expressed concern about potential overreach into states' rights issues.