Mellon v. Delaware, Lackawanna and Western Railroad Company was a United States Supreme Court case that addressed the issue of whether a state court had the authority to enjoin a federal court from hearing a case. The plaintiff, Mellon, had filed a suit in a federal court against the defendant, Delaware, Lackawanna and Western Railroad Company, for damages resulting from a train accident. The defendant then filed a suit in a state court seeking an injunction to prevent the federal court from hearing the case. The Supreme Court held that the state court did not have the authority to enjoin the federal court from hearing the case. The Court reasoned that the state court lacked jurisdiction to interfere with the proceedings of the federal court, and that the federal court had exclusive jurisdiction over the case. The Court further held that the state court's injunction was an unconstitutional interference with the federal court's jurisdiction. The Court concluded that the state court's injunction was void and that the federal court had the authority to proceed with the case.

In the case of Mellon v. Delaware, Lackawanna and Western Railroad Company, Justice Harlan delivered a dissenting opinion in which he argued that the majority’s decision was wrongfully based on an interpretation of state law rather than federal law. He argued that since this was a suit between citizens of different states, it should have been decided according to federal common law instead. Furthermore, he asserted that if Congress had intended for state laws to be applied in such cases then they would have explicitly stated so when passing legislation related to interstate commerce. In conclusion, Justice Harlan believed that the Court should not rely solely on Pennsylvania's statutory provisions but also consider other relevant sources such as general principles of commercial law or prior decisions from other courts when deciding similar cases involving interstate commerce disputes.