Ex Parte The Milwaukee Railroad Company is a United States Supreme Court case that dealt with the issue of whether or not an injunction could be issued against a railroad company for failing to comply with state laws. In this particular case, the Wisconsin Supreme Court had granted an injunction against the Milwaukee Railroad Company for failing to pay taxes and other fees as required by law. The U.S. Supreme Court reversed this decision on appeal, finding that such injunctions were unconstitutional because they violated due process rights under the Fourteenth Amendment of the Constitution. Furthermore, it was held that states did not have authority over interstate commerce and thus could not impose their own regulations upon railroads operating in multiple states without violating federal law. This ruling established important precedent regarding both due process rights and interstate commerce regulation which still stands today

In Ex Parte The Milwaukee Railroad Company, the Supreme Court was asked to decide whether a state court had jurisdiction over an action brought by citizens of another state against a railroad company. Justice Field delivered the dissenting opinion, arguing that while Congress has exclusive authority to regulate interstate commerce and railroads are part of such commerce, states have concurrent power with respect to intrastate transportation. He argued that this case involved only intrastate transportation and thus fell within the scope of state law. Furthermore, he contended that since it is not necessary for parties in an action involving only intrastate matters to be from different states or territories for diversity jurisdiction purposes under Article III Section 2 Clause 1 of the Constitution (the Diversity Clause), there was no reason why they should not be able to bring suit in their own home courts if those courts were competent tribunals. Therefore, Justice Field concluded that it would be improper for federal courts to interfere with proceedings already underway in a competent state court simply because one party happened to reside out-of-state at the time when suit was filed.