In the 1926 case of Napier v. Atlantic Coast Line Railroad Company, the U.S. Supreme Court ruled in favor of federal supremacy over state laws regarding railroad safety standards. The dispute arose when two employees of the Atlantic Coast Line Railroad were injured on duty due to alleged defects in locomotive equipment and sued for damages under Kentucky law which required additional safety appliances not mandated by federal law. However, the court held that states could not enforce regulations or standards that had already been addressed at a federal level through legislation such as the Federal Safety Appliance Act and Boiler Inspection Act - even if those state laws were more stringent than their federal counterparts. This ruling reinforced principles of preemption where conflicting state and local laws are invalidated by comprehensive national regulation.

In the dissenting opinion for Napier v. Atlantic Coast Line Railroad Company, Justice Stone argued that Congress did not intend to occupy the entire field of locomotive equipment regulation when it passed the Federal Boiler Inspection Act. He believed that states should retain their police powers to regulate in areas where federal law does not explicitly preempt them, especially when those regulations are designed to promote safety and welfare. In his view, Kentucky's requirement for a specific type of firebox was such a regulation; it was intended to protect train crews from excessive heat and smoke inhalation - hazards which were not addressed by federal law at that time. Therefore, he disagreed with majority’s decision striking down this state law as being preempted by federal legislation.