The LIA regulates "the design, the construction, and the material of every part of the locomotive and tender and of all appurtenances."

So we start with the assumption that the historic police powers of the States were not to be superseded by the Federal Act unless that was the clear and manifest purpose of Congress.

—the United States Supreme Court held that the LIA manifests the intention to occupy the entire field of regulating locomotive equipment.

The intention of Congress to exclude states from exerting their police power must be clearly manifested

R. Co., where the Court concluded that "the [Locomotive] Boiler Inspection Act... was intended to occupy the field."

In Napier, the United States Supreme Court found a Georgia statute that required locomotives to have an automatic fire door, and a Wisconsin statute that required locomotives to have a cab curtain, were preempted by the LIA because the statutes were directed at the "equipment of locomotives."

But when federal administration has made comprehensive regulations effectively governing the subject matter of the statute, the Court has said that a state regulation in the field of the statute is invalid even though that particular phase of the subject has not been taken up by the federal agency.

But obviously a state cannot, in respect to the regulation of interstate commerce, override the will of Congress.

In Napier, the Supreme Court decided broadly that "the Boiler Inspection Act has occupied the field of regulating locomotive equipment used on a highway of interstate commerce so as to preclude state legislation."